An Experimental Method to Investigatethe Optimal UV Filter for the Auger

Fluorescence Detectors

S. Maltezos and E. Fokitis

Physics Department,

National Technical University of Athens,Zografou Campus, GR-157 80 Athens, Greece

Abstract

A specific experimental method, in order to investigate the more suit-able UV filter to install in the Fluorescence Detector (FD) of the AUGERProject, is presented in this work. A prototype experimental setup hasbeen developed for this purpose. As detector, it uses an elementary cell,similar to the one used in the FD. A fast and sensitive photon countingsystem, is used for the light intensity measurements. The performance pa-rameters of the various candidate filters have been measured and comparedwith the values obtained from a simulation software, which is written ac-cording to the principles of the method. The capability of each filter toimprove the detection of the photons emitted from Extensive Air Showers(EAS) and reject the optical background, under the variable conditions ofthe background radiation, is studied in this work. Special measurementshave been taken concerning the transmittance also under the night-skybackground radiation (NSBR). A very good consistency between experi-mental results and that obtained from the simulation, has been found.

1 Introduction

The AUGER project [1] uses the hybrid detection technique for the study ofextremely high energy cosmic events. Two type of detectors will be used: theGround Detector (GD) array and the Fluorescence Detectors (FD). Each GDof the large array (1596 detectors), consists of a water Cherenkov tank, aimingto detect the muons, arriving in the ground from a shower developed in theatmosphere, induced by an extremely energetic cosmic ray event (EECRE). TheFD (eye), consists from 12 mirror units (telescopes) covering the full azimuthal

1

angle, aiming to detect the air fluorescence, mainly of nitrogen, radiation fromthe EAS. Totally 3 or 4 eyes are to be installed in the whole detector area.The combined detection of these two detectors types can improve the triggeringefficiency and the accuracy in the reconstruction of the cosmic ray event cascade.

In particular, each unit of FD detector consists from a spherical mirror com-bined with a Schmidt “corrector plate” and a pixel array of photomultiplier tubes(PMT) located at its focal plane. In the present work, we concentrate our inter-est in the specific optical filters, which are considered to be used in the FD. Theoptical filters must be able to select the UV light from the nitrogen atmosphericfluorescence, emitted from the EAS, during its shower development in the atmo-sphere. These filters will be placed either on the corrector plate of the telescope,or in front of the PMT window alternatively. The photons of the atmosphericfluorescence have to be detected under a variable NSBR, coming from varioussources such as, stars and galaxies, the moon, the artificial lights, zodiac lightand possibly aurora effects, and atmospheric night glow. An optical filter withhigh transmittance in the UV region, namely from 300 to 400 nm (band-pass),and very low one in the visible region, from 400 to 650 nm (band-stop), can im-prove the signal-to-noise ratio, especially in the high background level conditions.

The investigation of the optimal UV filter is based at several criteria, whichmainly focus in two directions. In the first direction, the performances under theexpected signal conditions (i.e. the distance and size of the shower) and underthe background variation (i.e. intensity level and spectral profile), plays the mainrole. The signal to noise ratio of the pixel detector (PMT), achieved by usingan optical filter, has to be higher than an acceptable minimum value, in order tocontribute efficiently to the trigger logic adapted in the FD for a valid EECRE.Therefore, any filter candidate which increases the probability of (true) trigger,can by definition increase the “aperture” of the FD detector.

In the second direction, the optical and mechanical properties of the filters,in combination with the cost per unit area, must be acceptable and not exceedthe specified constraints. As a result, the search for the best filter leads to theneed of making the correct compromise between the obtained performance andthe cost. This can be performed after a systematic study of each available filter.

In the present work, we try to give an answer to the question: which is the im-provement in the “aperture” of the FD detector using a particular optical filter?For this, it is necessary to determine the dependence of the minimum detectablesignal in terms of the filter performance parameters. Afterwards, the FD triggerefficiency should be calculated in a wide range of NSBR, whose statistical vari-ations have to be taken into account. For this objective we did measurementsboth in the laboratory and in condition of actual NSBR.

The effect of incidence angle spread on the filter performance parameters, hasbeen studied following a specific methodology.

The analytical description of the method is discussed in details in the section2, while in section 3 we present the entire experimental setup. The measurement

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procedure and the up to now obtained results are presented and analysed insection 4. In section 5, the simulation technique and the obtained results arepresented. In section 6 the conclusions and the further prospects are discussed.

2 Analytical description of the method

2.1 The integrated detection

Introducing the present method, some definitions and formulations are presentedand discussed. Although, they have initially been presented in Ref. [2, 3], wepresent here the complete formulation in more detail. Let us consider the spec-tral distribution of the photon flux per unit time from the radiation detectedfrom a single photo-tube (PMT). Thus, let S(λ) the signal spectrum and B(λ)background spectrum, over the spectral range from λ1 to λ2 (namely 280-650nm). Also, let Q(λ) be the quantum efficiency of the PMT photocathode, as afunction of wavelength. The total photon rate entering the PMT from an EAS,s, and the total photon rate entering the PMT from the night-sky radiation, b,can be written:

s =∫ λ2

λ1

S(λ)dλ (1)

b =∫ λ2

λ1

B(λ)dλ (2)

We define also Sn, Bn, as the normalised acceptance integrals (counting rates) ofthe PMT, assuming a total radiation equal to the unity in both cases. The in-troduction of these quantities facilitates the theoretical formulation. Thus, theycan expressed as follows:

Sn =∫ λ2

λ1

Q(λ)S(λ)

sdλ =

1

s

∫ λ2

λ1

Q(λ)S(λ)dλ (3)

Bn =∫ λ2

λ1

Q(λ)B(λ)

bdλ =

1

b

∫ λ2

λ1

Q(λ)B(λ)dλ (4)

In the case of using a filter of transmittance T, the above quantities become:

S ′n =

∫ λ2

λ1

T (λ)Q(λ)S(λ)

sdλ =

1

s

∫ λ2

λ1

T (λ)Q(λ)S(λ)dλ (5)

B′n =

∫ λ2

λ1

T (λ)Q(λ)B(λ)

bdλ =

1

b

∫ λ2

λ1

T (λ)Q(λ)B(λ)dλ (6)

Therefore, the total photon rates accepted by the PMT become correspond-ingly:

S = s.Sn =∫ λ2

λ1

Q(λ)S(λ)dλ (7)

3

B = b.Bn =∫ λ2

λ1

Q(λ)B(λ)dλ (8)

and therefore, the expression of the counting rate of the incident direct beam con-tains an intensity factor s (or b), and also the factors Sn (or Bn), which depend onthe spectral profiles and the spectral sensitivity of the PMT. The correspondingquantities of the transmitted beam, using an optical filter in the light beam, arecalculated as follows:

S ′ = sS ′n =

∫ λ2

λ1

T (λ)Q(λ)S(λ)dλ (9)

B′ = bB′n =

∫ λ2

λ1

T (λ)Q(λ)B(λ)dλ (10)

2.2 Improvement of signal-to-noise ratio

The signal-to noise ratio, S/N, of a PMT operating in the photon counting mode,have been studied by several authors, and is generally given by the following for-mula [4, 5]:

(S/N) =S√

S + 2B(11)

where S is the net signal rate and B is the overall background rate (backgroundplus dark counts).The corresponding quantity for the transmitted beam (through the optical filter)is written:

(S/N)′ =S ′

√S ′ + 2B′ (12)

According to the equations from 7 to 10 and substituting the expressions ofS, S ′, B, B′, we extract the following equations:

(S/N) =sSn√

s.Sn + 2bBn

(13)

(S/N)′ =sS ′

n√sS ′

n + 2bB′n

(14)

The ratio R = (S/N)′/(S/N) is defined as the “improvement factor” of thethe filter and it expresses the improvement of the S/N when using the filter, com-pared with that in the case of the direct beam. Using the above equations, this

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factor is written also:

R = (S ′

n

Sn)/

√sS ′

n + 2bB′n

sSn + 2bBn(15)

It is obvious that the ratios S ′n/Sn and B′

n/Bn represent the detection effi-ciency of the signal and the background correspondingly. Defining these quan-tities as Es = S ′

n/Sn and Eb = B′n/Bn, they can also be considered as useful

performance parameters of the filters; in this case the factor R is expressed interms of them and of s, Sn and b, Bn as well:

R = Es/

√sSnEs + 2bBnEb

sSn + 2bBn= Es/

√√√√(s/2b)SnEs + BnEb

(s/2b)Sn + Bn(16)

Alternatively, we can use the following expression which is usefull in the caseof experimental measurements:

R = Es/

√SEs + 2BEb

S + 2B= Es/

√√√√(S/2B)Es + Eb

(S/2B) + 1(17)

As it is obvious, the factor R depends on the parameters Es and Eb whichcharacterise the behaviour of the filter in the presence of signal and backgroundrespectively. Also, it depends on the ratio x = S/2B, but not on their specificvalues S and B. Consequently, R is a function of the variable x which could bevaried from 0 to ∞. For x � 1 the R tends to an expression where the backgroundis the dominant radiation. So, in the resulting equation we use the subscript b:

Rb = Es/√

Eb (18)

For x � 1 the R tends to an expression where the signal is the dominantradiation and for this we use the subscript s:

Rs =√

Es (19)

In all the other cases, the R is a function of the variable x with the Es, Eb,playing the role of parameters. For the simulation, these parameters can also becalculated, according to the integrals from 3 to 6.

An experimental measurement of the above parameters can be easily per-formed by the presented apparatus, because of the integrated detection of theradiation over all wavelengths. Therefore, an experimental curve for the R(x)

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can be obtained, varying the values of the ratio x. Experimentally, this can bedone, by simply changing the high voltage of the nitrogen discharge lamp. Theintegration of R(x), in the range where where R(x) ≥ 1, is an indicator of thefilter performance in detecting weak and strong signals as well.

Any other optical component which is inserted between the radiation sourceand the detector, must be taken into account in the fore-mentioned integrals, seealso in the next sections 3.2 and 5.2.

It can be noted, that the insertion of any optical component with a quiteflat spectral transmittance in the light beam, has not any certain effect on theabove parameters, due to the factorizing of the constant transmittance value inthe corresponding equations.

2.3 Influence of the incidence angle on the performance

An incidence angle variation implies an influence on the filter performance pa-rameters. This comes from the modification of the spectral transmittance of thefilter under various incidence angles. In the integrated detection of the radiation,the overall effect has to be determined.

Experimentally, it is necessary to produce a beam with the specified spreadof the incidence angle. In the simulation, the determination of the overall effectcan be done by including the dependence curve of the incidence angle on theperformance parameters. Such a curve can be obtained, experimentally, by ourapparatus, using the goniometer to rotate the filter. From the knowledge ofthis curve, its possible to calculate the overall effect following the methodologydescribed below:The incidence angle variation of the fluorescence or background radiation, isassumed to follow a uniform distribution. Its mean value will be biased (greaterthan zero), because of the pixel array camera in the central region. The rangedepends on the placement of the filters.

The density probability function in this case is: Dθ = 1/∆θ, where Dθ =θ2 − θ1 is the maximum range of the variation. This corresponds to a standarddeviation is σθ = ∆θ/

√12.

The integrated signal of the radiation, using a filter, will be the mean valueover the range of the angle variation, according to the following well-known for-mula:

S ′ =∫ θ2

θ1

S ′(θ)Dθdθ (20)

Substituting the Dθ and dividing with the direct (in normal incidence) signal,S(0), we obtain:

6

S ′/S(0) =1

∆θ

∫ θ2

θ1

S ′(θ)S(0)

dθ (21)

which is written also:

Es =1

∆θ

∫ θ2

θ1

Es(θ)dθ (22)

In the discrete form, needed for the calculations, the mean becomes:

Es =1

M

m∑i

Es(θi) (23)

where, Es is the mean of the detection efficiency, and M is the total number ofdiscrete samples.

In the same way, the mean Eb of the parameter Eb is correspondingly:

Eb =1

M

m∑i

Eb(θi) (24)

Consequently, the overall effect of the incidence angle variation on the im-provement factor R, is expressed by the Eq. 16, replacing the parameters Es andEb with those calculated above:

R = Es/

√√√√(s/2b)SnEs + BnEb

(s/2b)Sn + Bn(25)

2.4 Trigger threshold of the FD

A cosmic ray event produces an EAS in the atmosphere, whose the time durationis typically 60 µs. The role of the first level trigger unit is to identify a valid eventwithin a relatively short time measurement period. As described in the Ref. [6],an integration time of 1 µs is optimal to detect a valid event with a high enoughsignal-to-noise ratio (S/N) of each elementary detector. A minimum value ofS/N, let n, has to be set for the detection limit. It is acceptable, that a value(S/N)min = 5 allows to distinguish the signal from the noise (with 5σ). Thus, thePMT/filter cell has to be able to assure at least such a minimum performance ofdetection. The role of the optical filter in this critical operation is to enhance, asmuch as possible, the S/N under a wide range of background radiation conditions.

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The above condition is expressed according to the formula 14:

sS ′n√

sS ′n + 2bB′

n

≥ (S/N)′min = n (26)

This inequality can be written also in terms of the performance parametersEs, Eb:

sSnEs√sSnEs + 2bBnEb

≥ n (27)

After some calculations we obtain a quadratic equation, in the limit of equal-ity, in regard to s:

s2 − n2

SnEss − n22bBnEb

S2nE

2s

= 0 (28)

Solving this equation, we extract the minimum value smin:

smin =n2

2SnEs(1 +

√1 +

8bBnEb

n2) =

n2

2S ′n

(1 +

√1 +

8bB′n

n2) (29)

We observe that the obtained solution, smin, for a given value of n, is a func-tion of the performance parameters and also of the quantities Sn, Bn (or S ′

n, B′n)

and b, which could be varied during the FD operation. The Sn is expected to havesmall variations, mainly due to atmospheric scattering, given the stability of thespectral the fluorescence emission. The quantities Bn and b play the main role inthe latter. The background level, b, will vary up to several orders of magnitude,mainly due to the presence of the moon phase and the various atmospheric effects,while the Bn can have smaller variations induced modification of its spectral pro-file. In the variation of smin, we distinguish systematic and statistical variations.The systematic variations have to be studied separately depending on their origin(i.e. moon phase, aurora etc.). This statistical variations, during a long period,could be include periodical and random components. The statistical variationscould be included in the expression of smin (Eq. 29), estimating a mean valueand a standard deviation (STD) of a Gaussian distribution in each case. In thesebackground conditions, a Monte Carlo simulation is needed to include the abovevariations for a further study of smin. We present this MC study in the nextsection 5.2.

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3 Experimental setup

An experimental setup, which allows the direct determination of the filter per-formance parameters, is based on the principle of the integrated detection, asdescribed in section 2.2. Towards to this goal, we implemented a such setupwhose the principal ideas are discussed briefly below.

Firstly, the transmission measurement of the filter is performed measuring theintegrated intensity of the signal and background radiation, along the specifiedspectral region. These measurements can be held either for signal or backgroundalone or for a mixture of the two beams. Thus the behaviour of the filter isstudied in a direct way with requiring short time.

Secondly, in the transmission measurements, various incident angles can beselected using a collimated beam. The obtained angular dependence can be usedin the simulation. By further analysis, the effect of a beam with angular spread,which is the actual case in the operation of the FD, is obtained.

Third, some specific measurements of the background spectra (diffused night-sky radiation) and spectral transmittance of the filters, can be performed com-bining the operation of a grating spectrometer.

3.1 The mechanical and optical system

A schematic diagram of the entire experimental setup is given in Fig. 1. Weused the mechanical support of a spectrometer (PASCO model SP-9268) whichhas been modified to operate in UV region [7]. Its angular resolution is 0.1o. Asa source for obtaining nitrogen fluorescence, we use a nitrogen (N2) dischargelamp, attached to the input collimator. The beam passes through a slit 300 µmwide. The collimated beam is achieved using one lens from LiF (transparent upto UV region), 30 mm in diameter.

A side window PMT from Hamamatsu, type 931B with bialkali (Bi) photo-cathode, is installed at the focal plane of the exit lens of the spectrometer. It hasa gain of 7.3x106 for a HV=1000 V, anode dark current 5 nA and it is coated witha wavelength shifter (p-terphenyl) on its entrance window in order to extend itsspectral sensitivity down to about 280 nm. It is noted that this PMT has verysimilar characteristics with that of the type XP3062 from Philips [8], which isone of the candidate detectors. The existence of the p-terphenyl coating in theused detector, causes a slight effect in measuring the detection efficiency of thefilters, compared to that by using the PMT XP3062. This effect is studied bythe simulation.

The nitrogen lamp and the PMT as well are located inside light tight en-closures. The filter can be located in front of a metallic plate which has alsoa circular diaphragm of diameter of 8 mm (D2), in order to exploit mostly thecentral region of the lenses. The collimated beam passes through a light tightenclosure (dark box, DB), where the filter can be located or be removed. The

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measurements of the incidence angle dependence can be performed turning thegoniometer disk (GD) in a angular range up to 60o.

For the measurements with the background radiation, a beam splitter, typepolka 38105 from Oriel, with a flat spectral transmittance in the range of thetest (280-650 nm). It is installed inside the dark housing, under 45o angle. Thebackground radiation enters from a 8 mm hole (W1). The entire optics assuresa limited observation solid angle of 2.052 deg.2, a value close the correspondingof the FD detector cell. The additional diaphragm (D3), is used only for thetransmittance measurements.

An “artificial background” can be produced in the laboratory using a halogenlamp for the continuous component and a mercury (Hg) and a sodium (Na)discharge lamp to achieve the discrete components (spectral lines). It has beenalready evaluated, aiming to perform measurements of the filter performanceparameters. The advantage of using the “artificial background”, is the possibilityto study the filter behaviour under a wide range of background conditions.

A grating monochromator (noted as GMD), model 77250 from Oriel, as anoptional, can be positioned in front of the background entrance, when the spectraltransmittance of the filters and the background spectra measurements have to beperformed. A fused silica lens (f=200 mm) is used for the focusing of backgroundradiation into the entrance slit. The wavelength resolution of the GMD is namely0.5 nm and the focal length 125 mm over a range from VUV to near IR.

The effects of the atmospheric absorption [9], called as “reddening”, and the“corrector plate” material, which will be used in the FD, are important on thefilter investigation. The above effect on the performance parameters of a filter(Es, Eb and R) must be taken into account. This can be done by inserting theappropriate pieces of materials, or the material which mimics the effect, in thebeam path (i.e. beside the diaphragm D2, see Fig. 1).

For the “reddening” emulation of the nitrogen spectrum, we used a holloworthogonal box from fused silica walls, filled with water. A number of dropletsof coloured liquid have been injected in order to emulate and standardise severalatmospheric conditions. For the experimental measurements we emulated theatmospheric transmittance along the path referred in Ref. [9].

3.2 Discussion of the systematic errors

The main sources of the systematic errors may be considered the following:

1. Chromatic error of the lens affecting on the angular spread.

2. Light tightness imperfections of the dark housings.

3. Non-uniformity of the spectral transmittance of the lens and the beamsplitter.

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The first, causes an imperfection in the collimated beam less than 0.2o. Itscontribution in the counting measurements is negligible. The second, affects onthe background measurements and is of the order of the dark counts, that is,about 20 cps in average. Thus the corresponding contribution is 5 % in thevery low intensity measurements, but less than 0.005 % in the regular higherintensity levels. The third, affects mainly in the UV region and has an estimatecontribution less than 0.1 %. Therefore, the overall systematic error is estimated5 % in the low intensity levels and 0.1 % in the high ones.

3.3 Photon counting system and its performance

For the light intensity measurements in the above setup, we decided to use thephoton counting (or digital mode) in the PMT operation. This decision hasbeen obtained in order to achieve high stability in a wide intensity range andhigher accuracy in low light levels, compared with DC mode, see also Ref. [4].Indeed, high intensity levels of the direct beam are required in the case of filtertransmittance measurements in the band-stop, where the transmitted light beamis expected to have very low intensity.

As shown in the Fig. 1, the electric current pulses from the PMT anode areconverted to an amplified voltage pulse in the unit AM which is a preamplifierof the type AD2 from Thorn-EMI. The same unit contains also a discriminationcircuit with variable threshold level and dead time 25 ns. The voltage pulsesproduced in ECL, are converted to TTL standard by an appropriate unit of thetype ET1 from Thorn-EMI as well, operating up to 50 MHz. The pulse countingis performed by the unit CT (“Counter-Timer”), which has been designed andconstructed in NTUA [10]. The latter is able to count pulses within the selectedtime interval, using a 32-bit (4x8 byte) binary counter, operating up to 100 MHz.The time interval, ∆t, is selected by on-board jumpers, in the range from 1 msto 60 s. In our measurements we used a time interval 0.2 s for the differentialdetection (spectra) and 1 s for the integrated detection. For the latter case, weused ∆t=1 s, in order to be able to record the statistical distribution. Greatervalues could be used to reduce the statistical uncertainties.

A PC type computer performs the data transfer, the processing and the con-trol of the grating monochromator (GMC) as well. This is done by a programwritten in Quick Basic, using also a commercial interface card, type PCL-818HG[11], which performs A/D, D/A conversions and I/O digital signal transferring.The GMC operates in a full automatic way using a DC motor for scanning theselecting spectral range each time. The wavelength reading is done using a highprecision potentiometer (WL reading line in figure 1). The velocity of the gratingrotation and the position encoding are always under control from the PC.

The stability performance of the nitrogen lamp was measured by recordingthe counting rate distribution within a certain statistics in a rate level of 1000kcps. The resulting STD of the obtained Gaussian distribution, is 8 kcps, or

11

0.8 % in regard to the mean value, while the theoretical value for pure statisticsin pulse counting, which tends to 1 kcps, or 0.1 % (100/

√N) [12]. Therefore,

the distribution broadening is mainly caused by the lamp fluctuations, that is,of the order of 0.8 %. The statistical error, taking 10 samples, is estimated

δNstat =√

N/10, where N is the measured mean counting rate.Concluding, the overall statistical error in using the nitrogen lamp is estimated

0.25 %, while in measuring the night-sky background, the typical rates were muchlower (i.e. 4240 cps, see in the next sections), and thus, the error is 21 cps (or0.5 %), given the high stability of a such radiation.

The detection limit,Nmin, of the photon counting system can be estimatedfrom the formula Nmin =

√2Ndc [4], where Ndc is the dark counts. In our case,

the dark counts was 18 cps in average and, as a result, the detection limit is 6 cps,which shows the high sensitivity of the system in the absence of other additionaleffects.

3.4 Determination of the incident photon flux

The actual photon flux, which provides the measured counting rate in our detector(PMT), could be determined experimentally using calibrated sources. Althoughthis is in our future planning, we perform also an approximate calculation, basedon the PMT characteristics and on the used optical configuration, for comparisonwith the values for the NSBR, given in the literature.

The photon flux Nph, can be extracted from the counting rate Nct, in thesame integration time, using the following formula:

Nph =Nct

0.48QeCeAθ2m

(30)

where Qe is the average quantum efficiency of the PMT in the detection ofthe NSBR, Ce is the collection efficiency constant (taken equal to 0.8), Aθ2

m isthe observation solid angle, where the effective area A can be calculated from the8 mm PMT window. The factor 0.48 is used for taking into account the beamsplitter, while the angle θm is only 2.05o due to the beam collimation.

The error of the above approximation is estimated to be of the order of 15 %,mainly due to the uncertainty of the collection efficiency and the dependence ofcounting rate with the PMT high voltage, in a range ± 50 V along the “plateau”region. In uncertainties, the effect of aging in the PMT performance have beenneglected.

12

GMC

Inp. collimator

Computer PC-386

DC motor line

SC

L3

A/D-D/A

GMC position

and DIO

CT

WL reading

W1

BSD2

D3

DB

Con.

D1

PMT

GD

L2L1

DataBus

OFSupportGoniometer

Out. collimator

AM

HV PS

Bus

SignalRadiation DCR

HV PS

N 2 lamp

BackgroundRadiation

Figure 1: Schematic diagram of the experimental apparatus for measuring the per-formance parameters of the filters. OF, optical filter; BS, beam splitter; D1,D2,D3beam diaphragms; GD, goniometer disc; DB, dark box; GMC, grating monochro-mator; L1,L2,L3, lenses; AM, amplifier; DCR, discriminator; SC, signal condi-tioning ECL to TTL; CT, counter-timer.

4 Experimental procedure and results

4.1 Integrated radiation measurements

The measurement of the performance parameters is achieved by taking the count-ing rate in the direct beam (without the filter) and afterwards of the transmittedbeam (with the filter). In each measurement, 10 samples are used for countingwithin the time interval of 1 s, and the average value is calculated. For the signal,we selected high intensity levels for the direct beam (i.e. 1000 kcps) to minimisethe statistical errors. The selected high voltage of the PMT, according to the“plateau” curve, was in the range 1050-1150 V, while the supply voltage of thenitrogen lamp was varied from 500 to 800 V. In this method, it is necessary tohave a stability in the intnsity of the nitrogen lamp.

In the Tables 1 and 2, the obtained results for the tested filters are shown. Theused significant digits correspond to the achieved accuracy, taking into accountthe systematic and statistical uncertainties, as described in the correspondingsection.

13

Experimental performance results

Normal beam “Reddened beam”Filter type S(kcps) S ′(kcps) Es S(kcps) S ′(kcps) Es

Hi-Res 1003 570 0.568 280 140.0 0.500M-UG2 1003 511 0.509 280 132.5 0.473ZC &R 1003 351 0.350 280 107.7 0.382

Table 1: The obtained results in the laboratory using the nitrogen discharge lamp.To achieve the “Reddened” beam we used an appropriate absorbing optical mate-rial in the UV region. The dark count rate was 0.027 kcps and has already beensubtracted in the presented values.

Experimental performance results

“Reddened beam”+“Corrector plate”Filter type S(kcps) S ′(kcps) Es

Hi-Res 705 344 0.488M-UG2 705 326 0.462ZC &R 705 278 0.394

Table 2: The obtained results combining the use of absorbing material for the“reddening” and of PMMA-GS 218 (10 mm in thickness) for the “corrector plate”effect. The dark count rate was 0.020 kcps has also been subtracted.

4.2 Transmittance measurements

The spectral transmittance in the band-stop region of the filters, was performedfor the first time with a reasonable accuracy. For this, we used high intensitylevels (i.e 37-1000 kcps) from a halogen lamp, measuring also the transmittedbeam passing through the filters, see Fig. 2.

Due to the low intensity levels of the transmitted beam, the statistical fluctu-ations are visible in the plot. After digital filtering, the most of the random noisehas been rejecting, exhibiting more information. The uncertainty of the obtainedvalues mainly comes from the measured counting rate of the transmitted beamNt, which varies from 15 to 165 cps, Therefore, the relative error is the squareroot of Nt, that is 8 % for the higher transmittance values, while is 27 % for thelower one.

All the tested absorption filters show a very low transmittance value (about0.001 %) along the range 430 to 600 nm, while a wide transmittance peak isobserved at about 650 nm (red region) with a peak value of 0.6 %. The inter-ference filter, ZC & R, is shown a different behaviour. Some peaks, with values

14

440 460 480 500 520 540 560 580 600 620 64010

−4

10−2

100

Wavelength (nm)

Tra

nsm

ittan

ce (

%)

HI−RESM−UG2 ZC & ROCJ

440 460 480 500 520 540 560 580 600 620 64010

−4

10−2

100

Wavelength (nm)

Tra

nsm

ittan

ce (

%)

Figure 2: The obtained spectral transmittance of the filters Hi-Res, M-UG2 andZC & R in the band-stop region in a semi-log scale (upper plot). The same curve,as it is becomes smoother by digital filtering, is shown in the lower plot, where wepresented also the simulated curve of the filter OCJ for comparison reasons. Theexperimental uncertainties are discussed in the text.

from 0.05 to 0.7 %, are present in the range from 430 to 545 nm, in contrary tothe absorption filters mentioned above. In the red region the transmittance ismaintained less than 0.01 %.

4.3 Measurements under the night-sky background radi-ation

In order to obtain the most representative experimental value of the quantity Eb

for the various filters, it was necessary to perform the measurements under thenatural background, preferably during a moon-less night. For this, we transportedthe apparatus outside the city of Athens, to avoid the radiation of the artificiallights. The selected region was Eretria city, in Evia-Greece, about 40 km northfrom Athens in direct direction. The reason is that, from there the view of Athensis sufficiently covered by the mountain Parnitha (1400 m), in combination with

15

Experimental results of NSBR

Place 1 at 21:30 Place 2 at 20:15Filter type B(kcps) B′(kcps) Eb B(kcps) B′(kcps) Eb

Hi-Res 6.88 0.715 0.104 4.24 0.459 0.108M-UG2 6.88 0.684 0.099 4.24 0.428 0.101ZC &R 6.88 0.675 0.098 4.24 0.424 0.100

Table 3: The obtained results under the natural background. The dark count rate,was 0.007 kcps and has already been subtracted in the presented values.

availability of infrastructure for electricity to power the apparatus by countryhomes.

The installation has been done in two different places, “Magoula” (noted by1) and “Kastri” (noted by 2) in Fig. 3. Two different orientations were selectedin the above places with the criterion of minimising the artificial lights. In theorientation used in the place 2 (45 o E), only small villages exist in long distances.The milk-way was close to the same direction and visible by eye. The zenith anglewas about 72o (close to the optical axis angle of the telescope mirror of FD).The measurements have been taken during two successive cloudless nights (4and 5/12/99), under the following atmospheric conditions: Barometric pressureP=1023.5 hPa, Temperature T=14.2 oC, and Relative Humidity (RH)=73.7 %.

In the Table 3 we summarise the obtained values. It must be noted thata “corrector plate” material, like the PMMA-GS 218, was not necessary to usebecause its effect in the background measurements can be considered as negligible.

The artificial light components have been studied in details, measuring thenight-sky spectrum from our laboratory, 6 km from Athens centre, shown in Fig.4. It is evident that some strong spectral lines are present. The entire structure,mainly contains the lines of mercury (365, 404.7, 435.8, 546.1 nm), the Blue-Violet lines of sodium (449.4, 449.8 nm) and also the continuous component,which is increased almost linearly in average. Thus, the most of the intensity isconcentrated in the region above 400 nm. A similar structure was expected tobe present also measuring in Eretria, but in a lower degree. In the latter case,the continuum is expected to be a summation of artificial plus natural radiation,dominated by NSBR (air-glow and galactic).

4.4 Measurements in various incidence angles

As we mentioned in the previous section 2.3, the dependence curve of the inte-grated transmitted light from the incidence angle, is necessary for the determi-nation of the overall effect on the performance parameters, Es and Eb. For this,we performed such a measurements in various incidence angles for the filters ZC

16

10 km

2

1

ATHENS

ERETRIA

AMARINTHOS

HALKIDA

OROPOS

W

S

N

E

EVIA

EVOIC GULF

SARONIC GULG

1400 m

PARNITHA

ATTIKI

Figure 3: Topographical diagram, representing the region where we performed themeasurements of the night-sky background. The exact places and the orientationof the entrance window (arrows at places 1 and 2) are also shown.

& R and MU-G2, from 0 to 60o in steps of 2.0 ± 0.1o with a zero uncertaintyof ±0.5o. The direct beam was measured first, while in each angular positionwe were measured only the transmitted beam in order to avoid extra systematiceffects. The obtained curves of the Es(θ) and influence factor f = Es(θ)/Es(0),after “spline” interpolation, are shown in Fig.5. The errors are of the order of0.5 % in both data sets.

As we observe in the plot, the obtained curves of ZC & R filter (solid line)show some variations due to the transmittance curve structure. It is known, thatthe effect of the incidence angle on the spectral transmittance of an interferencefilter (thin-film multi-layer on a substrate), is a small non-linear shift of theentire curve towards the lower wavelengths [2, 3]. As a result, the overall effectdepends on the movement of the cut-off edge. The other shown curve (dashedline), corresponds to the filter MU-G2, while a similar curve has been obtainedfor the Hi-Res filter. For this type of filters the dependence appears smoother.

17

300 350 400 450 500 5500

0.5

1

1.5

2

2.5

3

3.5

4x 10

−3

Wavelength (nm)

Ligh

t int

ensi

ty (

a.u.

)

Figure 4: The night-sky spectrum taken from the laboratory, corrected by thegrating efficiency and the quantum efficiency of the PMT and normalised to theunity.

5 The simulation technique

5.1 Methodology

The simulation software is based in the formulation, presented in the previoussection 2.2. The calculations have been done in discrete form and as a resultthe integrals have been replaced with summations. We have taken into accountthe quantum efficiency curve of the PMT, the transmission coefficients of theatmosphere (from Ref. [9, 13]) and the spectral transmittance of the material,PMMA-GS 218 from Rohm with thickness 10 mm, which is considered as acandidate solution for the “corrector plate” of the FD. The experimental data,taken for the various incidence angles, have been included in the simulation aswell. The software has been written in the code of MATLAB environment [14].

In the simulation, some commercial and simulated optical filters have beenincluded and studied in details:Below we show a list of the filters, using a practical mnemonic code:

18

0 10 20 30 40 50 60 700

0.2

0.4

0.6

0.8

1

Incidence angle (Deg.)

Influ

ence

fact

or f

0 10 20 30 40 50 60 700

0.1

0.2

0.3

0.4

0.5

0.6

Incidence angle (Deg.)

Det

ectio

n ef

ficie

ncy

Es ZC & R

MU−G2

Figure 5: The experimentally obtained curves of the detection efficiency as afunction of the incidence angle for the filter ZC & R (solid line) and MU-G2(dashed line) are shown in the upper plot. In the lower plot the influence factordependence curve is shown for relative comparisons.

1. Hi-Res: absorption filter, from KOPP, used in the experiment Hi-Res, withdimensions 57x25 mm2 and thickness 1.1 mm.

2. M-UG2: commercial absorption filter, from Schott1, with dimensions 51x51mm2 and thickness 3.2 mm. This filter has been tested and compared withother commercial filters, see Ref. [15].

3. ZC & R: commercial interference filter from ZC & R2, with dimensions51.5x51.5 mm2 and thickness 3.3 mm. This filter is of a high quality stan-dard product, but has not been designed to have sufficiently wide transmit-tance curve along the UV region.

1Schott Glass, Germany2ZC & R Coatings, USA

19

300 350 400 450 500 550 6000

20

40

60

80

100

Wavelength (nm)

Tra

nsm

ittan

ce (

%) Hi−Res

MU−G2 ZC & RU−330 OCJ IDEAL

300 350 400 450 500 550 6000

20

40

60

80

100

Wavelength (nm)

Ligh

t int

ensi

ty (

a.u.

) N2 spectrum NSBR spectrum

Figure 6: The transmittance curve of the filters used in the simulation (upperplot).In the lower plot, the reddened nitrogen fluorescence and the NSBR spectraare shown as well.

4. U-330: commercial absorption filter, from Hoya3, relatively very expensive.

5. OCJ: simulated interference filter, from OCJ4, designed according to therequested specifications. It seems to be a very promising UV filter, as isconcluded in this work.

6. IDEAL: hypothetical ideal filter with T=0.96 in the region from 280 nm to400 nm and T=0 above 400 nm. Included in our studies, for comparisonreasons, to show the performance limits.

The transmittance data, mostly in the band-pass region, have been takeneither from the manufacturers (for M-UG2 and U-330) or from experimentalmeasurements with a double beam spectrophotometer [16] in UOA 5 (Hi-Res

3Hoya, Japan4Optical Coating Japan5University of Athens

20

and ZC & R). The above data are consistent with these obtained in our single-beam spectrophotometer, and have smaller systematic errors for relatively hightransmittance values (i.e. grater than 5 %). The data of Hi-Res filter are alsoin agreement with these of Ref.[9]. However, small statistical errors in the abovetransmittance curve donnot significantly change the following results from thesimulation analysis, which pertain to the overall filter performance in a widespectral range of the radiation. In Fig. 6, we present the mentioned transmittancecurves of the filters, used in the simulation, in the same plot. In this figure, wealso present the nitrogen fluorescence spectrum, as it has been modified by the“reddening” effect and the use of the “corrector plate”. The NSBR used in thesimulation is presented as well.

In the band-stop region, we used the data obtained from our grating spec-trophotometer for the filters Hi-Res, MU-G2 and ZC & R. Concerning the rest offilters (apart of the IDEAL filter), we obtained the data by digitising the availabletransmittance graph, provided from the manufacturers. However, these data havenot the desired accuracy, or show a transmittance value equal to zero in most ofthe spectral region. All the mentioned data are interpolated using the “spline”algorithm, in such a way to obtain a unified and more detailed wavelength scalein all of cases.

As night-sky background radiation we have taken that from the Ref.[17] incombination with data from Ref. [18], referred in a moon-less night, interpolatedboth in linear scale.

Comparative Table

Experimental SimulatedFilter type Es Esr Esrc Es Esr Esrc

Hi-Res 0.568 0.500 0.488 0.589 0.522 0.509M-UG2 0.509 0.473 0.462 0.515 0.480 0.471ZC &R 0.350 0.382 0.394 0.356 0.404 0.408

Table 4: The comparison of the experimental performance parameters with thatextracted from the simulation. The subscript, r, indicates the case of using the“reddened beam” and the, rc, the case of using also the “corrector plate” material.

5.2 Results of simulation

By the simulation we have studied the performance parameters and the improve-ment factor R, as they are affected by the use of “reddening”, the “correctorplate”. The effect of the incidence angle spread on the performance has been alsostudied for the two main types of filter. Furtherly, the minimum detectable sig-nal, has been calculated in order to conclude about the behaviour of the filters on

21

Comparative Table

Experimental SimulatedFilter type Rb Rbr Rbrc Rb Rbr Rbrc

Hi-Res 1.73 1.52 1.48 2.33 2.07 2.02M-UG2 1.60 1.50 1.45 2.09 1.94 1.91ZC &R 1.11 1.21 1.25 1.35 1.53 1.54

Table 5: The comparison of the experimental improvement factors Rb with thatextracted from the simulation. In the calculation of the experimental values weused the parameter Eb obtained from measurements in the more dark place 2. Thesubscript, r, indicates the case of using the “reddened beam” and the, rc, the caseof using also the “corrector plate” material.

the aperture of the FD. This is done according to the Eq. 29, where a reasonablemean value for b has been taken from Ref. [1, 6] and as a STD, we selected tobe 76 cps (70 % of the mean which is 108.5 ph/µs). For the Bn and Eb, as meanvalues, we selected the corresponding values of the filter under study, while asSTD for the Eb we assumed a value 0.00888 (10 % of the mean of the filter ZC &R, which is 0.0888). A normal incidence angle is assumed in these calculations.

In the diagram of the function smin = f(b), see Fig. 7, each point providesthe minimum signal which can be detected by the PMT/filter detector cell witha S/N=5. In the average conditions, the obtained values of smin are the fol-lowing: 215 for the Hi-Res, 231 for the MU-G2 and 269 for the ZC & R, all inph.µs−1.m−2.deg−2. The dependence with b is approximately linear in the pre-sented range, while the spread due to statistical variation of Eb is more intensein higher values of b.

Comparing the filters from the point of view of FD aperture, we consideredtwo hypothetical signals from a strong and a weak EAS (225 and 155 ph/µs),produced from the same distance. These provide a trigger to the FD with thefollowing average probabilities:In the strong EAS case: 98.0 % for the Hi-Res, 11.2 % for the M-UG2, 0 % forthe ZC & R and 100 % for the rest of filters. In the weak EAS case: 0 % for theHi-Res, M-UG2 and the ZC & R, 2.0 % for the U-330, 91.4 % for OCJ and 99.2% for an IDEAL filter.

Studying the influence of the incidence angle spread, over a certain time periodof operation in detecting the various showers, we assumed a uniform distributionin the range from 0 to 15o with a mean value 7.5o. The resulting overall influenceon the parameter Es, is very small (by a factor close to unity) for the filtersHi-Res and M-UG2, while for the ZC & R filter the factor is 1.03 (improvement3 %). The corresponding influence on Eb was not measured, but is expected tobe very close to the above values.

22

In the geometrical limit of the PMT camera of FD, the incidence angle rangebecomes 0 to 15

√2 = 21.2o with a mean value 10.6o. In this range, the the

influence for the filter ZC & R slightly changes and is expressed by a factor1.003, which is very close to unity and similar to the obtained for the absorptionfilters. In the case of placing the filters in the camera (in front of each PMTwindow), we expect a broad incidence angle range: from 22o to 40o with a meanvalue about 31o.

In our calculations, we also included the same type of our PMT, but withoutthe p-terphenyl coating. In this case, the results are referred also to the PMTof type XP3062, because both have have same photocathode and window. Theacceptance integrals Sn, Snr and Bn, where Snr pertains the case of the “red-dened” signal, using coated window (or PMT 931B) are: 0.233, 0.244 and 0.126correspondingly, while for the case of uncoated window (or PMT XP3062) are:0.225, 0.247 and 0.125. A difference less than 1.2 % in the acceptance integral isobserved between the mentioned PMT’s detecting the “reddened” signal.

For the “reddening” effect we used the transmittance curves of a “clear stan-dard atmosphere” model, in a zenith angle of 73.3o for a distant event 20 km inhorizontal direction and in hight 6 km, as described in Ref.[9]. In order to studythe behaviour of the filters under a worse atmospheric condition, we used also thetransmittance data, given in Ref. [13]. In this article, the atmospheric conditionsare classified in four types of air turbidity, from “D1” to “D4” with air mass 1.0(zenith angle 0). We selected the extreme classes: “D1”, which is referred to aclear atmosphere and the class “D4”, which is referred to an extremely turbidair case. We converted both to air mass 1.4 (zenith angle 45o) because the class“D1”, with such an air mass, corresponds to the realistic distant event, mentionedabove. The comparison results are presented in the Table 6.

The filters with high transmittance in the UV region, down to 300 nm, dis-play an improvement when the coating exist, due to the better matching of thespectral sensitivity curves. We also see, that the interference filter ZC & R showan improvement of the order of 2 % in the worse case of atmospheric conditionconsidered, in contrary to all the rest filters. Consequently, it assures a certainstability in detecting the nitrogen fluorescence radiation under a wide range ofatmospheric conditions. We also observe in the same table, that from the perfor-mance parameter values presented in the 8th column, the interference filter ZC& R becomes slightly better than the absorption filter MU-G2.

5.3 Discussion of the results

A comparison of the experimental values with that extracted from the simula-tion software was made with the required pedantry. Attention has been givenconcerning the effect of the imperfections, like effect of diffuse reflectance or pris-matic effects, because these should result a certain discrepancy. Indeed, in thefilter M-UG2 we observed the prismatic effect between its surfaces. This fact was

23

0 50 100 150 200 250 300120

140

160

180

200

220

240

260

280

300

Background b (ph/µs)

Min

imum

det

ecta

ble

sign

al s

min

(ph

/µs)

ZC & R

M−UG2

Hi−Res

U−330

IDEALOCJ

Figure 7: A diagram showing the points where the examined filters allow thedetection of the fluorescence radiation with a signal to-noise ratio (S/N) equal to5 and assuming the normal incidence angle. Two particular signals, from a strongand a weak EAS are shown using dotted lines as an example. These produce atrigger to the FD with average probabilities which differ between the filters.

more evident in the case of spectral measurements, due to the sensible geometryinside the monochromator. The deviation has been measured and found 25 µm(0.5 mrad in converging angle).

Under the above conditions, the results show that is possible to predict thefilter behaviour using the fore-mentioned simulation technique. In the Tables 4and 5, we present a summary of the obtained results. The values of the parameterEs differ from 1.2 to 5 % in the various filters. We also compare, the improvementfactor Rb, although this is not the more representative criterion of the filter overallperformance, as it is underlined in the conclusions. The average discrepancies ofRb in the three cases, -22.6 %, -23.4 % and -23.2 %, are very close together andare explained from the existence of the artificial light radiation measuring theNSBR, where higher value of Eb gives lower value of R (see Eq. 18). Comparingthe values of Rb and Rbr for the filter Hi-Res, with that presented in the Ref.[9],1.86 and 1.67, we see a relative difference of -7.0 % and -8.9 %, correspondingly.

24

0 500 1000 1500 2000 2500 3000 3500 4000

150

200

250

300

350

400

450

500

550

Background b (ph/µs)

Min

imum

det

ecta

ble

sign

al s

min

(ph

/µs)

ZC & R

M−UG2

Hi−Res

U−330

OCJ

IDEAL

Figure 8: A similar diagram with that shown in figure 7, but presented in a widerrange of background rate, as it is expected under the moonlight presence.

This discrepancy seem systematic and can be explained partially from the highercontribution of the artificial lights in the case of the mentioned reference. Also,it must be noted, that our measurements were taken in a direct way recordingthe integrated signal.

From the results, we conclude that the filter Hi-Res has a appreciable supe-riority in Rbrc (5.8 %), in the case where the “reddening” effect and the use ofthe “corrector plate” material are included, compared with the competitive filterM-UG2 which seems inferior under “conditions” considered. The difference inthe performance is greater when referring in the minimum detectable signal, smin

(7.4 % better), as shown in the Fig. 7. The simulated interference filter OCJ andthe absorption filter U-330 show excellent performance, with the first dominatingin smin 11.5 %. The performance of the interference filter ZC & R seems lowerthan the rest filters, but it appear a certain stability under higher atmosphericabsorption conditions. The relatively narrow transmittance band in UV of thisfilter, could be extended more in order to improve furtherly its performance.

The results under the night-sky background, make more explicit the effect ofthe artificial lights. Some dominant spectral peaks, mainly from mercury and

25

Simulation results

PMT 931B+coating PMT 931B or XP3062Filter type Es/D1 Es/D4 Diff.(%) Es/D1 Es/D4 Diff.(%) Esa/D4

Hi-Res 0.509 0.491 -3.5 0.490 0.472 -3.7 0.468M-UG2 0.471 0.457 -3.0 0.459 0.446 -2.8 0.442ZC &R 0.408 0.417 +2.2 0.422 0.431 +2.1 0.444U-330 0.685 0.673 -1.8 0.670 0.656 -2.1 -OCJ 0.771 0.761 -1.3 0.758 0.745 -1.7 -

IDEAL 0.791 0.784 -0.9 0.781 0.769 -1.5 -

Table 6: The obtained detection efficiency from the simulation in the cases ofusing the PMT 931B with and without p-terphenyl coating (similar quantum ef-ficiency with XP3062. Two atmospheric conditions, were assumed: D1 (clear)and D4 (extremely turbid air), which causes the shown percentage differences inthe particular filters. The subscript “sa” in the last column declares the includingof the overall incidence angle effect in the range from 0 to 15 degrees.

sodium are present, due to the corresponding lamps, in superposition with thecontinuous spectra from tungsten lamps.

From the measurements of the NSBR, we observe that the most obscured place2 (Fig. 3), the intensity of the direct radiation was about 62 % of that in theplace 1, under similar atmospheric conditions and measuring time. Therefore, thecontribution of the artificial lights in the first case was at least 61 % on the purenight-sky radiation. This contribution comes, mostly, from the cities of Halkidaand Eretria which participate correspondingly due to the selected orientation.According to the Eq. 30, the total photon yield in the place 2 calculated 368ph.µs−1.m−2.deg−2, which is about 3.4 times greater than that from Ref. [6],108.5 ph.µs−1.m−2.deg−2.

The intensity of the transmitted beam (or the parameter Eb as well), in place2 is 2.7 % greater in average than that in place 1 for the three filters. Thiscan be explained from the fact, that the higher percentage of the detected back-ground radiation comes from the band-pass spectral region of the filters, wherethe differences in the transmittance are indeed small. The fore-mentioned posi-tive difference confirm the quite effective rejection of the filters in the band-stop.Also, as it can be shown, it expresses the percentage of the artificial light radia-tion which was excess, in total, when measuring in the place 1. Indeed, measuringin the place 2 at the same orientation, we assure quite good night-sky conditions.

26

6 Conclusions and prospects

The performance parameters of the filters, measured by our apparatus, allowthe study of the variation of the minimum detectable signal. Consequently, theinvestigation of the optimal filter can be based in the criterion of triggering prob-abilities between the candidate filters.

We also measured the transmittance of the mentioned filters in the band-stopregion, with good accuracy and sensitivity, for the first time. The obtained finitevalues could introduce unpleasant background yield, mostly in the red region(“red leak”).

For a wide range of signal levels, a reasonable “Figure of Merit” for the filtercomparison is the integral of the improvement factor, Ri, that is, the area belowthe R(x) curve. For very low signals, which is the most critical case, somefilters might be rejected, due to missing triggering capability. In this case, theperformance parameter Rb (see Section 2.2, Eq. 18), is not a valid indicator, evenin the ideal filter case, because the approximation x � 1 is in contradiction withthe requirement to obtain a signal-to-noise ratio greater than 5. Thus, the signalin such a case is not detectable! The competitive filters can be compared withthe criterion of the provided triggering probability within a long-term operationperiod. The aperture of the FD is maximised by the filter whose operating pointsappear in the lower position in the diagram of Fig. 7. However, the systematicand statistical variations of the background radiation, play a significant role andhave to be studied experimentally.

From the experience in measuring the interference filters, such as the commer-cial filter ZC & R, and studying by the simulation the similar type filter OCJ, weconfirm the advantages of such filters concerning the design flexibility and highperformance possibilities.

Some prospects are discussed also briefly: The presented prototype appara-tus, could be further developed to achieve a motorised inserting-removing of oneor more filters movement perpendicular to the incidence beam. With such anautomation, faster and more accurate measurements for calibration of the realpixel detector can be achieved. In this case, long-term measurements of the night-sky background radiation could be performed. Results concerning the variations,mostly caused from the atmospheric effects, allow an extensive study which isuseful in studying the atmospheric transmission in the UV region.

One may also consider what could be done to characterise the performance ina grid of positions over the filter area, of optimal spacing, before they are installedin each of the telescopes of FD. This installation is also useful for studying theperformance of the R&D filters and a separate report on these results is underpreparation.

27

7 Acknowledgements

The authors would like to greatly acknowledge the cooperation of A. Geraniosand A. Petrides, in measuring the spectral transmittance of the tested filters andfor useful discussions, We thank P. Sommers for the very useful discussions andB. Khrenov and G. Garipov for helping in providing nitrogen lamp and theircooperation. Also, we are obliged to mention the help of J. Kleinfeller withsupplying the “corrector plate” material. We also acknowledge the cooperationof P. Moyssides in the selection and ordering the specific optical materials andE. Papantonopoulos for useful discussions. The authors also thank the graduatestudents J. Kapitsas and L. Karatzas, for their contribution in this work and inparticular for their design of the specific mechanical parts of the apparatus, inthe framework of their Diploma work.

We thank the Machinist K. Razakias for his excellent mechanical construc-tions. It is our pleasure to thank M. Basinas for the hospitality and his supportmeasuring in Eretria.

References

[1] AUGER Collaboration, “Design report of the Pierre AUGER Project”,March (1997).

[2] E. Fokitis, S. Maltezos, and E. Papantonopoulos “Optical Filter Designby the Simulated Annealing Method for the Fluorescence Detector of theAUGER Project”, Auger GAP Note 1998-022 (1998).

[3] E. Fokitis, S. Maltezos, and E. Papantonopoulos “Optical Filter Design byfor the Fluorescence Detector of the AUGER Project”,, Proceedings of theEuro-Conference on the Standard Model and beyond, electronically pub-lished by: Journal of High Energy Physics (JHEP), PRHEP-Corfu98/045(1999).

[4] Hamamatsu Photonics K.K., “Photomultiplier Tube-Principle to Applica-tion”,pages 67-68, and references therein (1994).

[5] Hamamatsu Photonics K.K., “Photon Counting using photomultipliertubes”, Technical Information, page 14 (1996).

[6] B. Dawson “Simulations of Noise Triggers and Trigger Optimisation in AugerFluorescence Detectors”, Auger GAP Note 1997-059 (1997).

[7] P. Moyssides, S. Maltezos, and E. Fokitis, “A VUV Prism Spectrometer forRICH Radiator Refractometry”, To be submitted to the Journal of ModernOptics.

28

[8] G. Matthiae and P. Privitera “Study of the Philips hexagonal PMT XP3062for the FD camera”, Auger GAP Note 1999-001 (1999).

[9] J. W. Elbert, “The HiRes Filter and Its Effect on the Aperture”, Proceedingsof Tokyo Conference on Techniques for the Study of Extremely High EnergyCosmic Rays (of M. Nagano), 232 (1993).

[10] S. Maltezos, M. Dris, and A. Filippas, “MAC-64 compatible interface cardfor time interval or pulse time rate measurements”, DELPHI Note: DELPHI91-115 RICH 47 (1991).

[11] ADVATECH, Europe GmbH, Zimmer Nr. 611, Willi-Becker Allee 11, D-40227 Dusseldorf, Germany.

[12] W. R. leo, “Techniques for Nuclear and Particle Physics Experiments”,Sprinnger-Verlag (1994).

[13] C. P. Jacovides, F. Timpios, D. N. Asimakopoulos, and M. D. Steven, “Urbanaerosol and clear skies spectra for global and diffuse photosynthetically activeradiation”, Agricultural and Forest Meteorology 87, p91-104 (1997).

[14] The MathWorks, Inc., MATLAB “The Language of Technical Computing”(1996).

[15] R. Cester, D. Maurizio, E. Menichetti, and N. Pastrone, “Evaluation ofcommercial UV glass filters for the Auger Prototype Fluorescence Detector”,internal report (1999).

[16] A. Geranios et al., “Optical Filter Transmittance Measurements by a Double-Beam Spectrophotometer”, in preparation.

[17] C. R. Jenkins, and S. W. Unger, “The night-sky spectrum from La Palma”,Royal Greenwich Observatory, Technical Note No 82 (1991).

[18] Galaxy spectrum from SAO FAST Spectrograph.

29