Nuclear Instruments and Methods in Physics Research A310 (1991) 379-384 Norlh-Holland

" N u c t u h I N I T R I I t m ~ l l

IN Pt lVI ICI RESEARCH

A self triggered intensified CCD (STIC)

Y. Charon a, p. Laniece a M. Bendali ", J.M. Gaillard h, M. Leblanc ", R. Mastrippolito ~, H. Tricoire a and L. Valentin ~

IPN Orsay, BP no. I, 91406 Orsay Cedex, France, and LPN Universit~ Paris Vii, 2 place Jussiet~ 75251 Paris Cedex 05, Fram'e,

h LAL Campus Orsay, 9100 Orsay. France

We are developing a new device based on the results reported previously of the successful coincidence detection of 1~ particles with a high spatial resolution [Y. Charon et al., Nucl. Instr. and Meth. A292 (1990) 179]. The novelty of the device consists in triggering and intensified CCD, i.e. a CCD coupled to an image intensifier (11), by an electrical signal collected from the II itself. This is a suitable procedure for detecting low light and rare events with high efficiency and high resolution. The trigger pulse is obtained from the secondary electrons produced by multiplication in a double microchannel plate (MCP) and collected on the aluminized layer protecting the phosphor screen in the I!. Triggering efficiencies up to 80% have already been achieved.

I. Introduction

Intensified CCD detectors have been widely used for many optical imaging applications because they offer high spatial resolution and good detection efficiency. When using such devices for rare and low light level events detection, one of the most important considera- tions is to decrease thermal noise contributions from both the I! and the CCD without loss of efficiency. In a previous application, we have demonstrated how to improve both factors, by driving the CCD in coinci- dence with a photomultiplier (PM) placed opposite to the ii input window [1]. This good performance trigger can only be used with transparent samples (like thin biological slides). Our goal is to realize a device which includes this type of CCD driving mode while removing the PM constraint from the trigger. For this, we are developing a self triggered intensified CCD (STIC). This paper presents in section 2 the principle of STIC and in section 3 the experimental setup. The first results concerning the triggering efficiency for single and multi photoelectrons (pe- ) level events are reported in section 4. The influence of various parameters like the MCP gain will also be discussed.

layer deposited on a phosphor screen. Due to the low accelerating voltage we use between the MCP and the II anode, part of the secondary electrons stop in this aluminized layer. The other part strikes the phosphor screen and produces light. Thus, when hit by photons the I1 delivers two types of signals:

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2. Principle

The basic principle consists mainly in collecting "quickly" an electrical signal when at least one p e - reaches a MCP hole. This signal is then used to start the CCD frame integration.

The anode of an II is made of a thin aluminized

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Fig. 1. Schematic representation of STIC: (a) Image intensifier with (1) photocathodc, (2) double microchannel plate, (3) phosphor screen with aluminized layer. (b) CCD system with (4) CCD, (5) CCD driving module. (c) Triggering system with

(6) amplifier. (7) integrator or threshold.

0168-9002/91/$03.50 ~. 1991 - El~vier Science Publishers B.V. All rights reserved V. BIOLOGY/CONDENSED MATI'ER

3tit) Y. Charon et aL / A self trlg~ert, d CCD fSTIC'J

A light signal which contains the spatial information and must he stored in the phosphor tmtil the start of C(' I ) frame integration. As the initial step is to clear the C('D (in step by step mode), this condition is fulfilh,'d by selecting a phospllor decay time wltich matches the CCD clearing p~riod.

An electric signal generated by collecting all the secondary electron charges via the anode aluminized layer. This signal is amplified and integrated to generate tile fast logic trigger for CCD integration.

Fig. i shows the trigger system and its connection to the intensified CCD. For each light event reaching the II, the eh,~trical signal has to be processed fast in order to collect the associated light on the CCD. The trigger- ing efficiency of STIC is determined by the level of the detectable electrical signal at which the system is oper- ational. Measurements of this triggering efficiency, a crucial STIC parameter, are reported in section 4.

3 . E x p e r i m e n t a l s e t u p

To characterize STIC performances, we have used the following material: - The light intensifier is a DEP [2] double MCP wafer tube prototype including a P46 phosphor screen coated with a 100 ,/k thin aluminized layer. Due to its high photon gain, we were led to reduce the usual voltage between the MCP output and the anode, in order to avoid the light feedback phenomenon. This configura- tion leads to the following voltage distribution:

PK-MCP, n = 0.03U. MCP, n-MCPou t = 0.32U.

MCP,,ut-anode = 0.65U.

U is the II global voltage (its standard value is set to 5 kV (800 V/MCP) that corresponds to an electron gain of ~ 10 s.

- The amplifier is a Texas Instruments UA733C dif- ferential video amplifier with a gain of 100. - T h e integrator is a fast integrating linear gate (Leblanc module). This device performs a charge in- tegration with a great sensitivity in an adjustable period (range of 10 ns up to a 100 its). The integrator delivers both a linear amplified integration signal and a TTL logic signal. It also includes gate, stretcher, delay gener- ator and anti pileup. - The photomultipfier is a XP 2264. set in front of the II output window and connected to a constant fraction discriminator (CFD). It analyzes the light emitted by the P46 phosphor screen and thus delivers an absolute event rating reference. The high photon gain of the 11 allows to set the CFD threshold to a value much higher than the PM thermal noise level, without loosing any event.

Due to the low amplitude of our linear amplified

signal, the electronic noise contribution (hf compo- nents) has been carefully studied. Therefore, all mea- surements have been performed in an electrically shielded environment. The disposition of all these com- ponents is presented in fig. 2.

Due to the low dynamic of the amphfied signal corresponding to single pe- , a charge integration dis- crimination is superior to a simple pulse height dis- crimination for the triggering efficiency. Therefore we have used a fast linear integrator (described above) with an integrating period adjusted to the amplified period width ("-150 ns). The module can be driven in two different modes: - an external mode which consists in using an external

signal to start the integration period; - an internal mode where the integration period is

started as soon as the amplitude of the incoming linear signal to be analyzed exceeds the module fast threshold value.

in order to measure accurately our triggering efficiency, we have studied the amplitude distribution of the col- lected electric pulse generated by single pe - event strik- ing the MCP. This spectrum has been obtained by using the Leblanc module in the external mode. The PM provides the required external signal with 100% ef- ficiency for single pc - events (see above). This integra- tor driving mode will be referred to as the PM Mode.

We then compared this PM Mode reference spec- trum to that obtained with the STIC alone. In that case, the integrator is driven in internal mode. That means that the ratio between the input pulse height and the module fast threshold value is a crucial factor for the triggering efficiency. Therefore, this threshold has been set as low as possible just above the amplifier intrinsic noise. To remove all electronic noise, we have required a signal from the PM to store only real events producing light on the phosphor screen. This integrator driving mode will be referred to as the Mixed Mode. In the following sections, we present the results obtained with both modes.

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Fig. 2. Experimental setup.

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4. Results

To test the efficiency of the device for single pe , we have used as single pc- source the thermal noise of the II photocalhode. The corresponding spectra for Mixed and PM Modes are shown on fig. 3. With the fast integrator threshold used the lowest amplitude pulses are lost in the mixed mode operation. For single pc- , the triggering efficiency defined as the ratio between the counting rates of the two modes reaches 80% for U -- 5 kV.

In view of biological applicationg, we have then tested the device with light produced by 3SS electrons crossing 10 pm NEI02 and SiY203(Ce) scintillator sheets. The subtracted noise spectra are shown on fig. 4 together with the single pe- spectrum. With SiYO a larger dynamic range, more than 10 pc-, is observed whereas for NE102 the spectrum is only slightly above that of single pe-s. Even though the light yield of SiYO is much larger, the triggering efficiencies are excellent for both scintillators with ~5S electrons. For a total voltage of 5 kV across the 11, which corresponds to 800 V across each of the MCPs, the triggering efficiencies are 87% and 98% for NE102 and SiYO respectively.

To study in more detail the properties of the system, we have measured the triggering efficiencies as a func- tion of the II high voltage and for different light source levels. As shown in fig. 5, the efficiency improves with increasing il voltages due to the gain increase. The signal becomes larger whereas the amplifier intrinsic noise level remains unchanged. On the other hand, the single pc- contribution, thus that of the thermal noise. becomes also more important, a source of increase for the STIC dead-time. The noise rate can be adjusted downwards by using a higher threshold at the price of an efficiency loss. Fig. 6 shows that loss as a function of the II voltage for different light source levels (NEI02 and SiYO). Even if a very stringent limit of 3 c / s were

imposed to the noise trigger rate thc detcctmn ¢ff,c~enc~, would still be 65~ for SiYO but only 8~ for NF.I(I2

5. Conclusion

Our preliminary results which have be~n presented show that STIC is a high performance and reliable detector for high resolution imaging of low counting rate and low light level sources. So far no special development efforts have been put into the video amplifier, it should be possible to improve the triggering efficiency by using an ad hoc low noise and large bandwidth amplifier. Using such an amplifier will prob- ably allow to replace the charge integrator by a simple voltage threshold. Another foreseen improvement is to replace the $20 photocathode by a bialkali one, thus lowering the thermal noise by at least one order of magnitude. We also plan to implement a system which allows to reduce both CCD clearing and integration times to a few I~S, thus vastly improving the signal to noise ratio of STIC zt. Several additional measure- ments are in progress to study in more detail the prop- erties of the device: linearity and saturation effects. spatial resolution, oplimisation of the voltages and alurninium layer thickness. The final step will consist in

=~ Such an improvement is made possible by the CCD fast clear operation (1 las duration), as developed and currently used for the UA2 scintillating fiber detector [3]. which allows a reduction of ihe integration time to a few tzs. A phosphor screen like PI5 would appear to have the right decay time properties and could be used for the anode. However. such a fastclear system is presently applicable Io CCDs with antiblooming, since the fastclear makes use of the existing antiblooming structure. To our knowledge, that concerns presently only small sensitive area CCDs ( - 0.5 cm 2) which are not large enough for our biological use of STIC.

Fig. 3. Single pc-

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V. BIOLOGY/CONDENSED MATTER

382 Y. Charon et al. / A self trOo{ered CCD (STIC)

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Fig. 4. Triggering signal spectra obtained for different light level sources: {a) photocathode thermal noise single pe spectrum; (b) NEI02 thin film (10 [xm) irradiated by 35S electrons; (c) SiY203(Ce ) thin film (10 p.m) irradiated by 35S electrons.

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Fig. 6. Detect ion efficiency with the influence of the global noise and for d i f ferent I! global vohage values (O: 4.6 kV. + : 5 kV, t3: 5.3 kV): (a) SiY thin film ( I0 lain) i r radia ted by ~SS c : (b) N e i 0 2 th in film ( 10 pm) i r radiated by S~S e .

V. B I O L O G Y / C O N D E N S E D M A T I E R

3L~4 Y. Charon et al. / A self triggered CCD (STIC)

demonstrating the performances of the STIC detector (the DEP wafer tube will be coupled to a THOMSON Ttl 7883 CCD) by imaging and quantifying 1~ labelled biological slides.

the double MCP P46 wafer tube prototype and for sharing their experiments on such devices.

Referencc~

Admowledgements

We wish to thank greatly the DEP company, and particularly T. Cochrane and L. Boskma, for supplying

11] Y. Charon et al., Nucl. Instr. and Meth. A292 (1990) 179. [2] Delft Electronischc Producten BV, Roden, The Nether-

lands. [3] R.E. Ansorge et al.. Nucl. Instr. and Meth. A273 (1988)

826.