Theoretical current response of silicon detectors to fast neutron beams in mixed n, γ fields

*Corresponding author.

Nuclear Instruments and Methods in Physics Research A 408 (1998) 503—510

Theoretical current response of silicon detectors tofast neutron beams in mixed n, c fields

M. Jung*, C. Teissier, P. Siffert

CNRS, Laboratoire PHASE, BP 20, 67037 Strasbourg Cedex 2, France

Received 12 June 1997; received in revised form 10 October 1997

Abstract

A theoretical approach is presented improving the knowledge on real time fast neutron dosimetry in intense fields. Thedetector is a reverse biased silicon junction operating at room temperature. The signal readout is a current measurementfor which the leakage current of the sensor sets the detector threshold in the range of 1 Sv s~1 dose rates. The dosemeterenergy response as a function of shielding, polyethylene neutron converters and diode characteristics has been computedto provide a tissue equivalent neutron monitor which is as insensitive as possible to the gamma photon background. TwoMonte Carlo codes, one for the fast neutrons (SEMIC) and for the photon background (MCGET), are used to calculatethe current response of the device irradiated by monoenergetic radiations (n or c) as well as neutron sources. ( 1998Elsevier Science B.V. All rights reserved.

Keywords: Dosimetry; Simulations; Real time; Silicon diode; High fluence; n, c fields

1. Introduction

Several years ago semiconductor detectors werefirst introduced as real time dosemeters to replacegas proportional counters for current measure-ments [1]. Nevertheless, the difficult task of manu-facturing tissue equivalent monitors, controllingwide energy range radiation fields justifies com-puter simulations to improve the knowledge on thedifferent parameters of these dosemeters. In thecase of mixed neutron—photon fields, the separationof c-ray and neutron signals makes the task evenmore difficult.

In this paper we are concerned with thedosimetry of large fluences of fast neutrons, forwhich a specific current readout is possible. Thiscurrent measurement monitors dose rates directly.It has to be as independent as possible of the everpresent c-ray background and also of any energyand angular diode dose equivalent response de-pendence.

The diode current produced by monoenergeticc-ray and neutron beams, measured with detectorassociated shielding components (SH), polyethy-lene (PE) neutron converters and detectors (D), iscomputed with appropriate Monte Carlo codes.

The detector D working at room temperature isa high resistivity N-type silicon junction biased at3.6 V, sufficient to deplete a thin layer of 30 lm.

0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reservedPII S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 2 0 5 - 8

For high sensitivity monitoring, a pulse heightacceptance cut-off helps to remove the c-ray back-ground signal. Otherwise, in the case of a currentmeasurement, which corresponds to the collectionof all generated charges, only a strong sensorshielding against photons can avoid their detection.This might increase only slightly the dosemeterresponse for neutrons up to 16 MeV. Only addi-tional secondary radiated charged fragments couldincrease the recoil proton signal from the polyethy-lene.

2. Dose response simulations

Papers on this subject have already been pub-lished [2,3], and only a few points will be repeatedhere. The signal from the semiconductor is a cur-rent from all the carriers produced by the chargedsecondaries emitted in the c-ray or fast neutroninteractions with shielding or hydrogenate mater-ial. The generated particles, either electrons orrecoil protons, crossing the diode D provide a num-ber of free carriers proportional to the energydeposited inside the junction, assuming that 3.6 eVare necessary to create an electron—hole pair. Thesecarriers, which are a function of the absorbed pri-mary irradiating beam fluence, move in the diodetowards the electrodes [4]. The geometry of thedevice is planar, the incident particles irradiatingthe front covered by the polyethylene foil andshielding material. The electrodes are fixed to op-posite front-back planes.

In both c-ray detection (MCGET2: diffusionpart of code MCGET) or neutron detection(SEMIC code), the computation starts witha Monte Carlo interaction process able to store theenergy lost by the fast electrons or recoil protonsinside the detector. The incoming parallel beam isassumed to irradiate uniformly the front area withthe possibility of varying the incidence angle.A fixed number of primary events is generated (upto 107) and the signal per incoming photon or fastneutron is modulated in a second step by the ap-propriate fluence to dose conversion coefficient.

The results are expressed in ambient dose equiv-alent units at 10 mm ICRU sphere depth as follows:for the c-rays the ICRU-47 values [5] and for the

neutrons the tables of Siebert and Schuhmacher[6], computed according to the new ICRU-51 andICRP-60 recommendations. More details on thecodes can be found in our previously publishedreports [2—4] for which some input data has beenupdated following Hubbell tables [7]. Note thatthe sampling methods differ for photons and neu-trons: the photon simulation [8] is a classicalMonte Carlo one, while the neutron interactionsare computed using a weighting algorithm similarto that of Ref. [9].

For the source irradiation simulations, the en-ergy of the incident neutron is chosen randomly,within the spectral limits, each interaction responsebeing immediately weighted by the source ampli-tude and by the dose conversion coefficient at thatenergy. The final result is normalized to get thecurrent measured per unit ambient dose equivalentrate. A similar process is used for the angular re-sponse computations to simulate a broad openincident beam irradiation.

The estimated current, I#, shown in the different

plots corresponds to the number of collectedcharges (coulombs) per second. It can be under-stood in two ways: either as the current of the diodeaccumulating the dose rate Sv s~1, or as the currentper 1 Sv s~1 normalized to 1 cm2 detector area. Inthis later case, for an area S

D, the diode D provides

a current J#equal to I

#SD.

2.1. Dosemeter characteristics study

Each simulated cascade is made for either anassociation of shielding material covering the sili-con diode or for a layer of polyethylene working asa proton radiator placed in contact with the de-tector. In the most common case, the diode D isa high resistivity 300 lm thick silicon N-type junc-tion, which, when biased at 3.6 V, produces a 30 lmdepletion width. In this layer the free minoritycharge collection contribution is complete, whereasthe free e` created inside the deep undepleted re-gion has to diffuse towards the junction to contrib-ute to the signal. Since the simulations are done toimprove the response of high dose equivalent ratemonitors, the current readout of the dosemeter hasno time limit set on the hole collection (for a practi-cal use the integration time is set to 1 ms). During

504 M. Jung et al. /Nucl. Instr. and Meth. in Phys. Res. A 408 (1998) 503—510

Fig. 1. Current (I#) plotted against neutron energy, calculated

for the D1 detector placed behind 0.01, 0.1, 1.0 and 2.0 mm ofpolyethylene (curves 1—4, respectively). The dots correspond tovalues calculated for the DS-30 detector.

that time, the deep minority carriers will contributeto the signal as a function of their lifetime.

For recording recoil protons the contribution ofthe deep undepleted region can be interesting, buton the other hand it is necessary to avoid thearrival of these deep minority carriers to reduce thec-ray background. A typical lifetime ¹

1of these

e` carriers is 100 ls (diode D1). We are also testingother possibilities, such as the response of less sensi-tive sensors with ¹

1"1 ls and others with 10 and

1 ns lifetimes, as well as that of a thin (30 lm),completely depleted silicon wafer DS-30. This lattercase sets a lower limit to the photon detectionsensitivity.

2.2. Fast neutron detection

Since one part of this report is concerned withthe question of how to increase the n/c response, letus first consider the maximum fast neutron detec-tion sensitivity to get an idea of the currentmeasurement one can tolerate for the photon back-ground.

In Fig. 1 the current I#, calculated for the most

sensitive diode (D1), placed behind 0.01, 0.1, 1.0 and2.0 mm thick polyethylene radiators, is plotted forneutron energies between 0.1 and 16 MeV. Thecomputations done with the SEMIC code [3] allowan analysis of a large number of events, the resultsshowing small statistical fluctuations. On thisgraph, the current I

#given by the least sensitive

diode DS-30 is also plotted. From these curves it isclear that for neutron energies above 7 MeVa slight discrepancy is observed, and that very littleinformation is lost when measuring only the re-sponse of the thin depleted Si region.

In Fig. 1, a maximum in the current is measuredabove 1 MeV neutron energy, the position againstenergy of the maximum increasing with the poly-ethylene thickness. The difficulty with this kind ofmonitoring is the strong energy dependent re-sponse even for the thinnest 10 lm polyethyleneradiator which shows a 92% fluctuation aroundthe mean response over the whole neutron energyrange. This fact gives little hope of obtaining a good“dose equivalent” monitor in the usual case whenthe energy spectrum of the incident neutrons isunknown, even when associating two polyethylene

radiators. In Section 4, a complementary type ofmeasurement is proposed. In any case, the mainpoint is the necessity to remove the c-ray signalfrom the detector. Keeping in mind that the max-imum current I

#for an exposure to 1 Sv s~1 neu-

tron dose rate is close to 200 nA, the response to anassociated c-ray background has to be much lowerif one expects to subtract it from the total current toremain with a clean neutron response.

In Fig. 2 the current is given for the detectors D1and DS-30 irradiated by PuBe and 252Cf sources.The curves are drawn against polyethylene thick-nesses between 10 lm and 2.5 mm. The mean en-ergy of each source is 4.63 and 2.21 MeV [3,9,10].The values, as in Fig. 1, show an increase of themeasured current with neutron energy and poly-ethylene thickness.

3. c-ray detection simulations

To discuss a way of obtaining a confident neu-tron signal in a mixed n, c field, we first look athow to reduce as much as possible the dosemeter

M. Jung et al. /Nucl. Instr. and Meth. in Phys. Res. A 408 (1998) 503—510 505

Fig. 2. Current (I#) plotted against polyethylene thickness, cal-

culated for the D1 detector irradiated by PuBe and 252Cfneutrons. The dots are values obtained for the DS-30 detector.

Fig. 3. Current (I#) plotted against c-ray energy, calculated for

the DS-30 detector shielded with 14.6, 36.5, 73.0 and 109.5 g/cm2

of a high atomic number filter SH (curves 1—4, respectively).


the DS-30 detector shielded with 14.6, 36.5, 73.0 and 109.5 g/cm2

of lower atomic number filter SH1

(curves 1—4, respectively).

sensitivity to photons. With this aim, computationsare done for a diode shielded against photons.

In paper [3], studies have already been done tolearn how to increase the n/c detection signal. Thecomputations were carried out for a few cm ofshielding, for which, the n/c signal ratio could befound to be larger than 1 if measuring irradiation(Gy s~1) dose rates. This is in accordance with theavailable experimental values from Ref. [11]. Sincethe dosemeter here is proposed for an ambient dosearea monitor for which the detector response to fastneutrons is reduced by dose equivalent coefficients,the efficiency of a gamma filter thickness increase isdiscussed. In this section, computations are pre-sented for two kinds of shielding materials: tin (SH)with a high atomic number and iron (SH

1) with

a lower one, to learn their behaviour againstphoton energy transparency. The values of Figs. 3and 4, calculated for the diode DS-30 and both highand lower atomic number SH and SH

1filters, show

the lowest diode c-ray response calculated for fourfilter thicknesses: 14.6, 36.5, 73.0 and 109.5 g/cm2.

It appears that the discrepancy is not large be-tween the responses from both materials, if thesmaller shielding thickness is considered for whichthe c energy detection threshold is however

lower for SH1. From both figures it becomes clear

that only the thickest filter is able to avoid thegamma detection below 1 MeV, with a factor twoinsensitivity gain in favour of SH

1material. The



different diodes of e` lifetimes ¹1"100 ls, 1 ls, 10 ns and 1 ns

shielded with 109.5 g/cm2 of SH1

material (curves 1—4, respec-tively). The dots are calculated for the DS-30 detector.

Fig. 6. Sum of two neutron channel currents (I#), plotted against

the neutron energy, calculated for the DS-30 detector behind0.01 and 2.0 mm polyethylene (curves 1—4). Curve 1 is the re-sponse of the 0.01 mm polyethylene.

necessity to use thick filters is certainly justified byconsidering that mostly gamma rays below 1 MeVare present in neutron surroundings. However, pos-sible higher energies can be found, for instance nearpower plants where 6 MeV c-rays cannot be ne-glected. However, due to limited computation time,the information is obtained with a finite number ofgenerated events and low probability responsescannot be excluded in the energy ranges apparentlyfree of gamma rays.

In the two previous graphs, the current calcu-lation was performed for the less sensitive thin30 lm silicon layer in order to look for the requiredfilters. In the next graph (Fig. 5), for the highestSH

1shielding, the influence of the diode character-

istics on the I#signal is discussed. The responses of

several kinds of diodes are drawn relative to thephoton energies. These diodes with 100 ls, 1 ls,10 ns and 1 ns minority carrier lifetimes (curves1—4, respectively) give decreasing c-ray responses.

However, it is clear that only a very short carrierlifetime (¹

1"1 ns) or a thin (DS-30) silicon wafer

gives similar responses, suppressing in an efficientmanner the c-ray background below 1 MeV. For

the maxima of the curves (3 or 4) in Fig. 5, the c-rayresponse is I

#"14.6 nA per Sv s~1, somewhat

lower than the maximum possible neutron re-sponse in the same energy range. This suggests thatan additional separate c-ray detection channelmight provide a background measurement thatcould be subtracted from that of an identicallyshielded associated neutron one. Also an evalu-ation of the photon dose above 1 MeV could bepossible within a 60% uncertainty level around themean value.

4. Improvement of the neutron dose measurements

We also studied if a more accurate dose measure-ment can be expected by associating weighted re-sponses of two different polyethylene radiators. Thenumerical weights’ optimisation is done with theMINUIT code from the CERN computer library.It uses a least-squares minimization of the fluc-tuations around the mean value of the summedweighted responses. In Fig. 6 the sum of theweighted response (curves 1—4) combined 10 lm


Fig. 7. Ratios (Rj) of the response calculated for 0.1, 1.0 and2.0 mm thick polyethylene radiators relative to the values ob-tained for the 0.01 mm polyethylene, plotted against the neutronenergy for the DS-30 diode (curves 2—4, respectively).

and 2 mm polyethylene radiators is plotted andcompared to the 10 lm response (curve 1). The asso-ciation of 95.6% of 10 lm with the 4.4% of 2 mmpolyethylene radiator neutron response allowsmonitoring the energy range from 0.1 to 16 MeV.

The dose equivalent measurement in this energyrange is uncertain by a large amount, perhaps 91%.This result is not an improvement compared to theprevious estimation from curve 1, even if it compen-sates the sensitivity drop above 3 MeV. It is clearthat one has to add spectral information to im-prove the dose measurement. So far, an approach ismade for getting information on the mean incidentneutron beam energy.

In Fig. 7, the ratios Rj of the responses obtainedwith three polyethylene thicknesses “0.1, 1.0 and2.0 mm” to that of 0.01 mm polyethylene are plot-ted. The ratios Rj remain close to 1.1 to within 10%below 2 MeV neutron energy and then increasemonotonically to different values.

In such a way, the Rj value defines two neutronenergy intervals:

1. the first one for neutrons up to 1 MeV (Rj closeto 1). In this energy range a mean current

I#equal to 5.3 nA cm~2 per Sv s~1 dose rate is

measured (Fig. 1) within $82%. This uncer-tainity remains close to this value over the wholeneutron energy range as discussed in Section 2.

2. the second one for neutrons above 1 MeV (Rjlarger than 1). In this energy range, there ispossible to estimate for a mean current I

#of

15 nA cm~2 (curves 1—4 from Fig. 6) the1 Sv s~1 dose rate with a $45% variation. Thisgives hope for improving the detection accuracyif the responses of two separate neutron chan-nels are processed numerically: one with a 10 lmand a second one with a 2 mm polyethylene foil.

In this case, also if Rj is much larger than 1, it iseven possible to read in Fig. 7 the mean fast neu-tron energy value. The energetic information re-ported in Fig. 1 (curve 4) allows to calculate thedose rate corresponding to the measured current.Such a treatment can give a very precise dosemeasurement, since as seen in Fig. 2 the currentI#from the fast component part of broad neutron

spectra is close to that of the monoenergetic neu-tron beams corresponding to their mean energy(Fig. 1).

5. Angular response dependence

The angular response variation for parallel inci-dent neutron beams is calculated for the DS-30diode and the thinnest polyethylene radiator. Thevalues are computed using the ambient dose equiv-alent angular coefficients from [12]. Fig. 8 showsa plot of the responses of incident beams at 30° and60°, normalized to that of 0° incidence. On the samegraph, the normalized detector response calculatedfor incident neutrons irradiating the front of thedevice within $75° is shown. This generatesa smearing effect for a spread-out opening anglewhich remains below 2.1 over the whole energyrange. The largest RS value, for neutrons below2 MeV is even less.

Fig. 9 compares, for the 10 lm polyethyleneradiator, the current response calculated for theDS-30 diode irradiated either by the normally inci-dent neutrons, or as in the previous graph, bya $75° open beam. One can conclude that the


Fig. 8. I#

ratios RS(h) plotted against the neutron energy, cal-culated for dose equivalent neutron fluences at 30°, 60° and fora $75° open beam, normalized to the current response at 0°.

Fig. 9. Current (I#) plotted against the neutron energy, cal-

culated for the DS-30 diode and a 0.01 mm polyethylene radi-ator. The values computed for an open beam (within $75°) arecompared to that of the normally incident ambient dose equiva-lent neutron flux.

Fig. 10. Detection thresholds (Dt) plotted against the neutronenergy, calculated for a 5 nA leakage current. The detector isplaced behind 0.01, 0.1, 1.0 and 2.0 mm polyethylene radiators(curves 1—4, respectively).

angular dispersion does not change the responsesubstantialy, since it increases the response at high-er neutron energies with an effect close to that of

the associated polyethylene device response seen onFig. 6 (curves 1—4).

6. Conclusion

A theoretical approach to real time fast neutronambient dose equivalent rate monitoring with sili-con diodes is presented. A sensitivity comparisonfor c-ray detection using similar dosemeters [1] ispossible with currents of the same level: 0.6 nA perR/min (i.e. about a few hundred nA per Sv/s). Theapplication of the current measurement withsemiconductors shifts their detection thresholds to-wards Sv s~1 dose rates, due to leakage currents ofseveral nA, as can be seen in Fig. 10. On this graphthe curves are drawn for a leakage current of¸#"5 nA per cm2 diode area. The lowest thre-

sholds computed for the diode D1 vary, dependingon the polyethylene thickness, between 26 mSv s~1

at 10 MeV (curve 4) and 250 mSv s~1 at 3 MeV(curve 1). Also, for the neutron energies below1 MeV, the region of most interest in controllingdose rates around power plants [13], the detectionthresholds are close to a few Sv s~1.


These diodes can tolerate high beam intensitiessince damaging improves the n/c signal, and even iftheir detection threshold is too high for shut downmonitoring, they can give information on the doserates attained during the power rise period. Thesedosemeters are also interesting for risk area as-sesment since large dose rates are possible in case ofaccidental irradiation. The detection sensitivity iscomparable to that of other solid state materialslike MOSFET for which detection field sensitivityfor photons is also around 1 Gy/s [14].

The difficulty in obtaining an accurate doseequivalent rate measurement over a wide unknownenergy range is obvious. An accuracy improvementis possible by associating the response of threesensors, two of them covered by 0.01 and 2.0 mmpolyethylene layers, respectively, from which onesubtracts the c-ray background measured by anidentical bare detector. This signal treatment be-comes possible by encapsulating the whole devicewith a thick shielding and using thin diodes asdiscussed in Section 3.

If the photon signal is negligible, a combinationof two radiator responses can provide spectro-scopic information on the mean neutron energyand thus allows improving the accuracy of the doseequivalent measurement.

References

[1] G. Jacob, G. Forcinal, J. Meuleman, Nucl. Instr. andMeth. 101 (1972) 51.

[2] M. Jung, M. Fasasi, C. Teissier, P. Siffert, Use of semicon-ductor detectors in personal dosimetry, 7th Int. Conf. onRadiation Shielding, Bournemouth, UK, 12—16 September1988, pp. 553.

[3] M. Jung, C. Teissier, P. Siffert, C. Raffnsoe, Fast neutronmonitoring with silicon detectors, STRAHLENSCHUTZ:Physik und Messtechnik, 26 Jahrestagung des Fachver-bandes, Karlsruhe 24—26 May 1994, vol. 1 1994, pp. 151.

[4] M. Jung, C. Teissier, P. Siffert, Radiat. Prot. Dosim. 51 (3),(1994) 157.

[5] International Commission on Radiation Units andMeasurements (ICRU) — Measurement of dose equivalentsfrom external photon and electron radiations. ICRU Re-port 47, Bethesda MD, 15 April 1992.

[6] B.R.L. Siebert, H. Schuhmacher, Radiat. Prot. Dosim. 58(3) (1995) 177.

[7] J.H. Hubbell, S.M. Seltzer, Tables of X-ray mass attenu-ation coefficients and mass energy-absorption coefficients1 keV to 20 MeV and for elements Z"1 to 92 and 48additional substances of dosimetric interest, Report USDepartment of Commerce, May 1995.

[8] M.K. Fasasi, Modelisation des Reponses du Silicium et duTellure de Cadmium aux Rayonnements Gamma et Neu-tron, Application a la Dosimetrie, Doctoral Thesis, Uni-versity of Strasbourg-France, June 1987. CRN/CPR, 87-04, n 236.

[9] M. Jung, Ch. Ott, Fast Neutron Code SAD1, Internalreport, Strasbourg, CRN/CPR-16, 1985.

[10] H.H. Knitter, Atom Energy Akte 22 (1973) 84.[11] I. Petr, J. Seda, B. Sopko, Properties of silicon photodiodes

in n-c fields, VIth Symp. on Neutron Dosimetry, Neuher-berg-RFA, 12—16 October 1987.

[12] B.R.L. Siebert, H. Schuhmacher, Radiat. Prot. Dosim. 54(3/4) (1994) 231.

[13] G. Wolber, Y. Guibbaud, R. Dollo, La Dosimetrie desNeutrons en Centrale Nucleaire, Problemes et Solutionsen 1995, Electricite de France, Comite de Radioprotection,15 mai 1995.

[14] J.M. Sanders, Rad. Prot. Dosim. 17 (1986) 303.


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Theoretical current response of silicon detectors to fast neutron beams in mixed n, γ fields