Gamma-rays and fast neutron responses calculations for personal electronic dosimetry purpose

*Corresponding author. Tel.: #33-388-10-6338; fax: #33-388-10-6293.

E-mail address: [email protected] (M. Jung).

Nuclear Instruments and Methods in Physics Research A 458 (2001) 527}533

Gamma-rays and fast neutron responses calculationsfor personal electronic dosimetry purpose

M. Jung*, C. Teissier, P. Si!ert

PHASE Laboratory, CNRS BP 20, 67037 Strasbourg Cedex 02, France

Abstract

Real-time dosimeters with small size N-type silicon diodes are proposed here for low-dose rate controls. Numericalsimulations are used to predict the responses of various associated "lters, neutron converters and sensors. The monitor isforeseen to work as a counter with acceptance cut-o!s set on each individual pulse height. Discussions are undertakenagainst the minimal outline necessary to reach convenient measurement accuracies in unknown gamma}neutron"elds. ( 2001 Elsevier Science B.V. All rights reserved.

Keywords: Real time; Room temperature; Personal dosimetry; Fast neutrons; Gamma-rays; Mixed "elds

1. Introduction

Electronic counters are particularly interestingas pocket dosimeters. Since room temperaturediodes produced from silicon junctions becameavailable, di!erent research teams have studied andproposed small size detectors for gamma-ray andneutron environmental survey measurements[1}5]. Our laboratory has, for many years, de-veloped computer tools and sensors [6,7] useful forradiation detection. This paper is an overview ofour numerical simulation studies. Results areshown for the responses against radiations, given

by di!erent detection channels including gamma"lters, neutron converters and low-biased siliconN-type diodes [8}10]. The dosimeter is foreseen forlow-dose rates control in mixed gamma and neu-tron "elds. Its signal output is studied as a digitalcounter, where the pulse-height acceptance cut-o!s`Ca separate the neutron and gamma detection.The diode responses are computed for incident fastneutrons sources and monoenergetic parallel be-ams in the energy range of 0.1 to 16MeV. Photonirradiations are computed for energies between20 keV and 10MeV.

In this paper, results are reported for a simulta-neous photon and fast neutron detector with ac-ceptable response accuracy for a minimal numberof detection channel output combinations. The nu-merical associations of channel responses are notonly envisaged to improve the dose measurementaccuracy, but also to assign the mean environ-mental beam energies [10,11].

0168-9002/01/$ - see front matter ( 2001 Elsevier Science B.V. All rights reserved.PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 9 1 4 - 1 POSTER SESSION

2. Detector channels and their calculated responses

In our previous reports we have extensively dis-cussed the discrepancies between photon and fastneutron electronic dosimeter sensitivity detectionlevels [6,7]. This property has been used to extractthe neutron signal from the photon background.To make clear the di!erent elements of the pro-posed monitor, we de"ne the so-called channels`CHa, and the responses as function of incidentradiations are simulated for di!erent channels.Each channel includes a 300lm thick, 2.75]2.75mm2 silicon diode. This diode is reverse biasedby 3.6V, which is a low value, convenient in pocketdosimetry. The diode emits a signal due to freecarrier motion inside the sensor, therefore the inci-dent radiation photon or neutron has to be con-verted into ionizing radiations capable of creatingan amount of free carriers inside the junction. Thisprocess is obtained by various "lters iron (Fe) andpolyethylene (PE) converting foils.

2.1. Numerical simulations

Two sets of codes, SEMIC and MCGET2, areused. They are based on Monte Carlo simulationsof the primary beam interactions inside stacked"lters or converters and the diode. They include thecharge collection during the integration time withthe possibility to set a digital acceptance cut-o! ontheir collected amount, expressed here as an equiva-lent energy deposited. The detector works asa counter, and it enables high-detection sensitivities.

For the silicon material in use, 3.6 eV of energyis necessary to produce a pair of electron}holecarriers.

The counter response is converted intoa dosimetric measurement by weighting, either im-mediately or afterwards (for monoenergetic parallelbeam irradiations), the response by the #ux to per-sonal dose equivalents at 10mm ICRU slab depth[12}14].

Throughout the report, the dosimeter responsesare discussed either in counts N

#per unit dose

accumulated by the detector (similar to that of1 cm2 area) or in n

#"N

#) SD (where SD is the

diode area). These values can be used to calculate

either the dose rate D5

(or d5"D

5)SD~1), which

gives a counting frequency equal to one count persecond (1 cTs).

2.2. Dosimeter channel dexnitions

The silicon diode is a high-resistivity N-typejunction. Due to the low bias only 30lm of siliconis depleted, where all the holes are assumed toinduce charges on the front electrode. The holes inthe remaining deep undepleted layer can di!useinto the junction and add their contribution to the"nal collection, depending on the integration time,hole lifetime and hole di!usion coe$cient. All oursimulations, which were completed for low- orhigh-dose rate monitoring, have shown that toimprove the neutron/gamma signal one has to sup-press the contribution from deep junction undep-leted layer to the "nal signal. It consists, in fact, ofusing the so-called diode DS instead of D1 [9,10].Also, it has been shown that light atomic shieldingsuppresses e$ciently the photon background sig-nal. Both restraints are convenient for the neutronmonitor device housing. They are, however, notoptimal for gamma-ray dosimeter measurements.Nevertheless, an attempt is undertaken to includea photon detection channel, for which low cut-o!sare studied against gamma energy range measure-ment accuracies.

In practice, three channel responses arecomputed separately (Table 1), their response asso-ciations leading to "nal so-called `Ma monitorproposals. One has to recall, that for photon chan-nel simulations, a 1.5mm aluminum "lter is in-cluded in front of the diode to reproduce electronicequilibrium. In all cases, the diode D1 response iscomputed for a 100ls hole lifetime and 1 ls integ-ration time.

Table 1 summarizes the counting e$ciencyN

#values at the normalization energy of di!erent

graphs.

3. Electronic digital signal acceptance discussion

In this section, the various channel responses arecalculated for monoenergetic parallel, normally in-cident beam irradiations.

528 M. Jung et al. / Nuclear Instruments and Methods in Physics Research A 458 (2001) 527}533

Table 1Channel de"nitions!

Channel Converter Diode C (keV) N#

per

} 0.01mm PE DS 0 0.104lSvCH1 0.01mm PE DS 300 0.078lSv} 2.0mm PE DS 0 5.853lSvCH2 2.0mm PE DS 300 4.320lSvM1 95.3% CH1#4.7% CH2 DS 300 0.275lSvM2 95% CH1#5% CH2 DS 300 0.329lSvCH3 0.36 g/cm2 Fe D1 20 1.255nSvCH1 housing 0.36 g/cm2 Fe DS 20 1.058nSv} 0.36 g/cm2 Fe D1 25 1.207nSv} 0.36 g/cm2 Fe DS 25 0.949nSv} 0.36 g/cm2 Fe D1 30 1.163nSv} 0.36 g/cm2 Fe DS 30 0.735nSv

!Summary of counting e$ciencies per unit doses calculated at the normalization energies: 14MeV for the neutrons, 660 keV (137Cs) forphotons. The di!erent values correspond to the plotted graphs.

Fig. 1. Comparison between upper pulse-height limits cal-culated against "lter thicknesses.

Fig. 2. Normalized to 14MeV neutron response of diode DScounting calculated for two cut-o!s: 0 keV (dashed) and 300keVand two polyethylene thicknesses.

Fig. 1 illustrates the minimal cut-o! necessary toremove the photon signal in channel CH3. Themaximum pulse-height values calculated for thischannel are reported as function of "lter thickness.The values for both diodes DS and D1 are com-pared. Obviously, only DS (excluding high atomicshieldings such as Sn) ensure a con"dent photonsignal transparency for cut-o!s around 300keV.

This high cut-o! does not drastically reduce theneutron dosimeter counting e$ciencies (Fig. 2 andTable 1), it only shifts the energy detectionthreshold above 400keV. For diode D1, the di$-culty appears for energies close to the 6MeVphotons that one "nds around power plants. Tohave a clean neutron signal in that case, largercut-o!s (larger than 700keV) must be introduced.

M. Jung et al. / Nuclear Instruments and Methods in Physics Research A 458 (2001) 527}533 529

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Fig. 3. Normalized to 137Cs response of shielded diode DScalculated against gamma energy for three cut-o!s.

Fig. 4. Normalized to 137Cs response of shielded diode D1calculated against gamma energy for three cut-o!s.

Table 2Summary table for: the mean counting e$ciencies SN

#T per unit dose with Sn

#T"SN

#T ) SD; the mean dose rate for 1 count/sec (1 cTs),

Sd5T"3600/Sn

#T

Channel Accuracy (%) Energy range (MeV) SN#T per Sd

5T for 1 cTs

CH1 73 0.45}15 0.25lSv 190mSv/hCH1 for c-ray 50 0.04}1.5 1.058nSv 45lSv/hM1 33 0.7}16 0.34lSv 140mSv/h

M1 28 0.75}15 0.35lSv 136mSv/hM2 30 0.7}16 0.38lSv 125mSv/hM2 24 0.75}15 0.41lSv 116mSv/hCH3 70 0.08}7 1.122nSv 42lSv/hCH3 #50/!65 0.08}6 1.157nSv 41lSv/hCH3 50 0.08}4.5 1.255nSv 38lSv/h

Such cut-o!s would have the disadvantage of losingthe possibility of neutron monitoring below theMeV level [10]. The diode DS is the most interest-ing in the neutron dosimetry, and we will discuss itslimitation when keeping its housing instead of D1for gamma-ray dose measurements.

Usually, the gamma-ray dosimeters work withlow cut-o! values just above the electronic noise toremain with low-detection thresholds. Also, goodaccuracy including 6 MeV photons is only possiblewith deep contribution from the diode D1.

Figs. 3 and 4 compares DS and D1 response vari-ations versus photon energy, the normalizationpoint counting e$ciencies N

#at the 662 keV 137Cs

energy are given in Table 1. The values are drawnfor three cut-o!s. A mask centered around 1 isadded, it de"nes a $50% accuracy intervalaround the normalization point. It could be shiftedto look for the most interesting energy interval.Di!erent con"dence bands are reported in Table 2.Clearly, the 20 keV threshold is a convenient cut-o!for channel `CH3a gamma-ray control. With diode


Fig. 5. Comparison of gamma-ray response for both diodes DSand D1 (C"20keV). Fig. 6. Normalized `M1a and `M2a (dashed) neutron response.

D1, it is possible to achieve an acceptable accuracyincluding 6 MeV photon detection. The response of`CH3a versus gamma-ray energy is shown onFig. 5, comparing DS and D1 counting e$ciencies.

4. Final outline discussion

Two main questions are essential: how manychannels are needed and for which accuracy?Throughout this section, we are progressively in-creasing the number of channels, emphasing thenew detection application. One has to keep in mindthat the `CH3a material is presumed for the wholedosimeter housing.

4.1. One channel **CH1++

The gamma-ray detection is three orders of mag-nitude larger than that of fast neutrons. If using CH1and two cut-o!s (20 and 300keV) one can simulta-neously measure, the gamma and neutron doseswith accuracies shown in Table 2. One has to re-member that channel CH1 is assumed to be coveredby CH3 material (0.36 g cm~2 covering 1.5mm Al).Also, that due to the large photon, neutron detec-tion e$ciency discrepancy (Table 2), the gamma-ray dose measured is `cleana against neutron signal(i.e. of the 0.01mm PE proton recoils).

4.2. Two neutron channels

What brings two neutron channels?

f Combination of two weighted channel responseassociations have shown the accuracy gain forneutron dose measurement (Fig. 6). However, thegain is not high. Especially, around power plants,where mostly neutrons below 2MeV are expected[15,16], there remains an uncertainty that cannotbe reduced. Two monitor responses (M1 and M2)have been already optimized [10,11]. The simula-tions were done for normal incident `M1a andcompared with large ($753) omnidirectionalrotating beam `M2a. A good detection accuracyis reached with two channels association, espe-cially for neutrons above 2MeV.

f Also, if one adds numerical signal treatmentalike ratios (Rj) calculations of responsesCH2/CH1 the energetic dependence is found tobe monotonously increasing even for angularvariations of the incoming beam incidence(Fig. 7). In Fig. 8, Rj ratios calculated for wideneutron spectra referred [11] have been reportedversus their mean energy value and comparedwith the previous plotted curves. There isa [10,11] good agreement between mean experi-ental energies and interpolated values from Rj


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Fig. 7. Ratios Rj against neutron energy calculated for di!erentangular irradiations.

Fig. 8. Comparison between monoenergetic and neutron sour-ces of the Rj ratios.

Fig. 9. Ratios R(H) calculated for both channels `CH1a and`CH2a against neutron energy.

ratio using precalculated curves. In that way, ifthe mean energy is estimated by this method,a more sensitive response can be extracted fromthe `CH2a response curve.

f Also, a di!erent angular response behavior ver-sus normal incident monoenergetic neutron be-ams is observed for CH1 and CH2 (Fig. 9).Therefore, di!erent orientations depending on

either CH1 or CH2 response variation indicatesthe surrounding neutron energetic range.

4.3. A third channel **CH3++, or what bringsa separate photon detection channel?

Table 2 shows that with a diode DS in CH3, onlygamma-rays below 2 MeV have a good detectionaccuracy. No improvement is expected, comparedto CH1 results, this additional channel becomesredundant. A real gain is achieved with diode D1 ina separate `CH3a, since 6MeV photon detection isincluded within #50% to !65% accuracy. Evena comparison with CH1 output, for the same20keV cut-o!, can provide important information,whether gamma-rays above 2 MeV are present ornot?

5. Conclusion

The interest in small room temperature diode assensors for real-time dosimetry has been estab-lished. Even with a rather low response sensitivity,it is, nevertheless, su$cient to give an alarm ina fast neutron contaminated environment [10,16].

Further, we have discussed the information gainwhen increasing the number of channels.


A single channel monitor `CH1a is not uninter-esting for both neutron and photon survey, ina classical environment, where most photons ener-gies do not exceed that of 137Cs. In this channel,a frequency of 1 count/sec monitors a mean gammadose rate equal to 45lSvh~1 (Table 2). If one caresabout the ICRP recommendations [17] "xing theannual dose to 50 mSv, a signal at every 8 s isa good alarm. The neutron detection sensitivity ismuch lower, and the diode emission can be used forshort-time irradiation controls, allowing the possi-bility to leave a dangerous environment [10,11].

Additional channels? If introducing numerical sig-nal combinations, multichannels bring interestinginformations on the mean energy as well forneutron as for photon control. The sum of weightedCH1 and CH2 outputs (Fig. 6) allows measure-ments between 0.75 and 15MeV with accuraciesbetter than 30% (Table 2). The dose rates (Table 2)are calculated for the 7.6mm2 diode area. This areais small, and especially for the neutron control it isnot uninteresting to think about larger detectionsurfaces.

References

[1] Y. Eisen, G. Engler, E. Ovadia, Y. Shamai, Z. Baum,Y. Levi, Tenth Workshop on Personal NeutronDosimetry, Accapulco, Mexico, 1983.

[2] T. Nakamura, M. Horiguchi, T. Suziki, T. Yamano,Radiat. Prot. Dosim. 27 (3) (1989) 149.

[3] Tetsuo Matsumoto. Radiat. Prot. Dosim. 35 (3) (1991) 193.[4] M. Barelaud, B. Dubarry-Chabanais, D. Paul, L.

Makovicka, J.L. Decossas, J.C. Vareille, Radioprotection28 (4) (1993) 387.

[5] T.O. Marshall, D.T. Bartlett, P.H. Burgess, C.S. Cranston,D.J. Higginbotton, K.W. Sutton, Radiat. Prot. Dosim. 34(1/4) (1991) 91.

[6] M. Jung, M. Fasasi, C. Teissier, P. Si!ert. Seventh Interna-tional Conference on Radiation Shielding, Bournemouth,UK, 12}16 September 1988, p. 553.

[7] M. Jung, C. Teissier, P. Si!ert, C. Ra!nsoe. StrahlenschuK tz:Physik und Messtechnik, 26 Jahrestagung des Fachver-bandes, Karlsruhe 1, 24}26 May 1994, p. 151.

[8] M. Jung, C. Teissier, P. Si!ert, Radiat. Prot. Dosim. 51 (3)(1994) 157.

[9] M. Jung, C. Teissier, P. Si!ert, Nucl. Instr. and Meth.A 408 (2}3) (1998) 503.

[10] M. Jung, C. Teissier, P. Si!ert, Nucl. Instr. and Meth.A 431 (1}2) (1999) 264.

[11] M. Jung, C. Teissier, P. Si!ert, SPIE, International Sympo-sium on Optical Science Engineering & Instrumentation,Denver, 18}23 July 1999, 3769, Report 27.

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

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

[14] International Commission on Radiation Units andMeasurements (ICRU), Report 47, Bethesda MD, 15 April1992.

[15] W.G. Alberts, J.M. Bordy, J.L. Chartier, C.R. Jahr,H. Klein, C.M. Luzik-Bhadra, H. Schuhmacher, B.R.L.Siebert, Radioprotection 31 (1) (1996) 37.

[16] G. Wolber, Y. Guibbaud, R. Dollo. ElectriciteH de France,ComiteH de Radioprotection, 15 mai 1995.

[17] International Commission on Radiological Protection(ICRP), Publication 60, Pergamon Press, Oxford, UK,1990.


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Gamma-rays and fast neutron responses calculations for personal electronic dosimetry purpose