Design and test of an albedo personal neutron dosemeter based on PADC detectors

Radiation Measurements 44 (2009) 972–976

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Design and test of an albedo personal neutron dosemeter based on PADC detectors R. Bedogni a, *, G. Gualdrini b, A. Esposito a, R. Mishra c, S. Tripathy c a

INFN–LNF, Frascati National Laboratories, via E. Fermi n. 40, 00044 Frascati, Italy ENEA-Radiation Protection Institute, Via dei Colli n. 16, 40136 Bologna, Italy c BARC-Bhabha Atomic Research Centre, Trombay, 400085 Mumbai, India b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 September 2008 Received in revised form 31 August 2009 Accepted 7 October 2009

Albedo dosemeters, usually based on thermo-luminescence detectors, have been frequently employed in personal neutron dosimetry. Their main advantages are the high sensitivity and the reduced angular dependence, whilst the disadvantages are the sensitivity to photons and the significant energy dependence of the response, which drops dramatically above 0.1 MeV. The present paper proposes an albedo dosemeter based on a planar PADC coupled with a 10B converter. This dosemeter shows good potentials in terms of photon insensitivity, limited angular dependence and high neutron sensitivity. Its use coupled with a dosemeter sensitive in the MeV region, should result in promising dosimetric performances. The response of the PADC based albedo dosemeter was characterized through irradiations in reference neutron fields of 241Am-Be, 252Cf and 252Cf(D2O) and Monte Carlo simulations with the code MCNP-4C. The paper presents the results of the experimental and computational studies and outlines the relevant dosimetric performance of the new albedo dosemeter. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Albedo Neutron personal dosimetry Chemical etching CR-39 PADC Monte Carlo

1. Introduction Since its introduction in the late 60s (Preston, 1968) the albedo dosemeter became, in diverse forms and designs (Piesch and Burgkhardt, 1985), one of the most popular neutron personal dosemeters. Nowadays, this type of dosemeter is still employed in many installations and dosimetric services (Draaisma and Verhagen, 2002; Schwartz and Eisenhauer, 2002). Compared with other fast neutron personal dosimetry techniques, the albedo dosemeters certainly show larger energy dependence of the response, which varies by orders of magnitude from the thermal to the MeV neutrons. By contrast, as shown by a trial performance test organized by EURADOS (Bordy et al., 2001), the variability of the response with the angle of incidence for the albedo dosemeters is lower than for nuclear emulsions or etched-track based dosemeters. This is due to the nearly isotropic spatial distribution of the thermalized neutrons. To detect the neutrons thermalized within the human body and back-scattered towards the dosemeter, 6LiF based thermo-luminescence detectors have been frequently used, due to a series of advantages such as the low cost, simple reading procedure, large response to thermal neutrons and reusability.

* Corresponding author. Tel.: þ39 0694032608; fax: þ39 0694032364. E-mail address: [email protected] (R. Bedogni). 1350-4487/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2009.10.002

Nevertheless such a configuration, using a combination of TLDs and track detectors, is affected by two important problems: 1) The large photon sensitivity of TLDs imposes the use of pairs of detectors with different photon and neutron sensitivity, such as the TLD-600 and TLD-700. In workplace fields with dominant photon component, this constitutes a very important source of uncertainty. 2) A very complex and expensive experimental equipment is needed, including the PADC etching and reading system plus a TLD reader.

As suggested by Luszik-Bhadra et al. (1993), these problems should be solved by using an albedo configuration where an appropriate 6Li, 10B or 14N converter is interposed between the body and a CR-39Ò or PADC (poly-allyl-diglicol-carbonate) track detector. The PADC will register the charged particles emerging from the converter as result of the (n,p) or (n,a) reactions induced by the low energy neutrons back-scattered from the body. The combination of this PADC based albedo stage with a PADC exposed to the direct neutron field should yield a dosemeter with highly improved energy dependence of the response. It is in fact well known that, whilst the response of the albedo dosemeter decreases with the neutron energy and becomes very poor above

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Table 1 Results of the simulations in terms of number of (n,a) reactions in the boron converter per unit incident fluence, termed r. This value is related to the dosemeter in the center of the irradiation array, corresponding to the center of the phantom face. The contribution due to the phantom-scattered neutrons (scattered fraction, SF) is reported in percentage. Source

0

20 2

r (cm ) SF

Fig. 1. The irradiation set-up. Nine albedo dosemeters are simultaneously exposed on a ISO water filled phantom.

Thermal 1 keV 10 keV 0.1 MeV 1 MeV 2 MeV 10 MeV

4.04 1.46 1.21 0.985 0.580 0.460 0.144

252

0.1 MeV, the response of the PADC (chemically or electrochemically etched) exposed to the direct neutron field shows a threshold at 0.05–0.1 MeV then increases with the energy, reaching a maximum at about 2 MeV (Bedogni et al., 2008). This work focuses on the albedo stage, proposing a configuration based on a planar PADC coupled with a 10B converter. The energy and angle dependence of its response were calculated with MCNP-4C (Briesmeister, 2000) and validated through comparison with experiments performed at different angles of incidence in reference neutron fields of 241Am-Be, 252Cf and 252Cf(D2O) available at the calibration laboratories of ENEA-Bologna (Bedogni et al., 2002) and INFN-LNF. The paper presents the results of the experimental and computational studies and a preliminary evaluation of the dosimetric performance of this albedo configuration, which is planned to be combined with a well established PADC based fast neutron dosemeter (Bedogni et al., 2008) to form a wide energy range neutron personal dosemeter. 2. Materials and irradiation tests The PADC detectors were produced by the Italian company Intercast Europe s.r.l. (www.intercast.it). The dimensions of the detectors are 2.5 cm  3.5 cm  0.14 cm. As a suitable boron converter, BE10 sheets from Kodak were selected. The boron, in the form of bound powder, contains 99% of 10B. The density of the converter is 1.45 g cm3 and its thickness 50 mm (Ferrarini, 2007). The boron converters were coupled to the detectors and irradiated in terms of personal dose equivalent, Hp(10), using a 30 cm  30 cm  15 cm ISO water filled phantom. The irradiation configuration, shown in Fig. 1, consists of an array of nine dosemeters simultaneously exposed on the phantom. Each dosemeter, as illustrated in the detail on the right of Fig. 1, consists of:

Cf(D2O) 1.31 Cf 0.499 241 Am-Be 0.369 252

40 2

r (cm ) SF

60 2

r (cm ) SF

r (cm2) SF

15% 3.76 95% 98% 99% >99% >99% >99%

14% 3.00

11% 1.82

89% 1.23 >99% 0.472 >99% 0.354

88% 1.00 >99% 0.411 >99% 0.308

86% 0.645 >99% 0.294 >99% 0.240

7.8%

81% >99% >99%

After being exposed and separated from the boron screen, the detectors were etched for 90 min in a 6.25 N KOH solution at 70  C. The read out procedure was performed using an in-house automated reader, obtained by upgrading the existing INFN-LNF reader (Bedogni et al., 2008). It consists of an epi-illumination microscope on which a 8 Mpixel CCD camera is mounted. This allows acquiring images with 3272  2469 pixels and 256 grey levels. Four 0.48 cm2 reading fields per dosemeter are analyzed, covering a total area of 1.92 cm2. The resolution is 2.5 mm/pixel. 3. Simulations The assembly shown in Fig. 1 was simulated in detail with MCNP-4C for the experimental irradiation conditions (neutron source and angle of incidence) described in Section 2. In addition, to predict the response of the dosemeter as a function on the neutron energy over a wide energy range, additional simulations with thermal neutrons and a series of mono-energetic neutron beams (1 keV, 10 keV, 0.1 MeV, 1 MeV, 2 MeV and 10 MeV) in normal incidence condition were performed. To simulate the real irradiation scenario, a plane parallel irradiation field investing the whole phantom face was used. The number of (n,a) reactions in the boron converter per unit incident fluence, indicated with r, was computed for each dosemeter, with

1) The ‘‘support PADC’’. This detector, glued to the inactive face of the converter, is only used to hold it. Such an arrangement allows manipulating the boron screen, which is very thin and intrinsically fragile. 2) The front detector registers the a particles emerging from the boron converter. Irradiation tests were performed in reference neutron fields of Am-Be (fluence-average energy 4.16 MeV), 252Cf (2.13 MeV) and 252 Cf(D2O) (0.55 MeV). For each field, angular exposures with angle of incidence 0 , 20 , 40 and 60 (with respect to normal incidence) were performed. The delivered value of Hp(10) was 1 mSv in all cases. The room- and air-scattered radiation was accounted using the shadow-cone technique for the point sources of 241Am-Be and 252 Cf, and the polynomial technique for the extended 252Cf(D2O) source (ISO, 2000). 241

Fig. 2. Total and phantom-scattered neutron spectra in the support PADC for an incident field of pure thermal neutrons. The total spectrum is normalized to unit incident fluence.

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Fig. 3. Total and phantom-scattered neutron spectra in the support PADC for an incident field of 241Am-Be. The total spectrum is normalized to unit incident fluence.

Fig. 5. Total and phantom-scattered neutron spectra in the support PADC for an incident field of 252Cf(D2O). The total spectrum is normalized to unit incident fluence.

stochastic uncertainty lower than 1% (one standard deviation). This quantity was assumed to be proportional to the track density per unit incident fluence measured in the PADC. Through the use of a ‘‘cell flagging’’ option, it was possible to separate the (n,a) reactions arising from neutrons belonging to the incident field from those originated by phantom-scattered neutrons. Moreover, the total and phantom-scattered neutron spectra in the ‘‘support PADC’’, i.e. at the entrance of the boron screen from the phantom front face, were determined. Table 1 reports, for the different energy and angles of incidence, the value of r obtained for the dosemeter located at the center of the irradiation array, corresponding to the center of the phantom face (dosemeter n. 5 in Fig. 1). The fraction of r due to phantom-scattered neutrons (scattered fraction, SF) is reported in percentage. In Figs. 2–5, the total and phantom-scattered neutron spectra in the ‘‘support PADC’’ are reported, for normal incidence condition, for the thermal, 241Am-Be, 252Cf and 252Cf(D2O) spectra, respectively. The volume where these spectra are calculated is the ‘‘union’’ of all nine support PADCs. From Figs. 2–5 it is clear that, apart from the pure thermal field, the thermal component in the support PADC is only due to the phantom back-scattering. Since the 10B(a,n) reaction has an E0,5 energy dependence, the dosemeter response for these neutron

fields is expected to be nearly totally due to the phantom backscattering, as demonstrated by the data in Table 1. Only in heavily thermalized workplace fields, here represented by the ‘‘pure thermal’’ field, the incident neutrons give a significant contribution to the dosemeter response. In view of applications in personal dosimetry, the knowledge of the energy dependence of the dosemeter response in terms of Hp(10) is essential. To obtain this function, the quantity r obtained for mono-energetic beams in normal incidence condition was averaged over the nine dosemeters and divided by the proper fluence-to-Hp(10) conversion coefficient (ICRU, 1998). This quotient, indicated with rH (cm2 mSv1) is reported in Fig. 6 as a function of the neutron energy. As expected, the response drops dramatically above 0.01 keV, confirming that the albedo configuration is mostly suited for low energy neutrons.

Fig. 4. Total and phantom-scattered neutron spectra in the support PADC for an incident field of 252Cf. The total spectrum is normalized to unit incident fluence.

4. Experimental validation of the model For an exposure to a given neutron field and angle of incidence, the track density (in cm2) of the nine dosemeters was measured with the reading technique described in Section 2, averaged and corrected for the room- and air-scattered radiation. The corrected average track density per unit personal dose equivalent, indicated with RH (cm2 mSv1), was compared with the corresponding simulated data, rH. Table 2 reports the results of the validation experiment. The experimental uncertainties (about 8%–9% for the

Fig. 6. Energy dependence of number of (n,a) reactions in the boron converter per unit Hp(10). The data refer to the average over the nine dosemeters under normal incidence irradiation. Lines are only eye-guide.

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Table 2 Comparison between the simulated data and the measured response of the albedo dosemeter. Experimental uncertainties are the standard deviations of the reading of the nine dosemeters summed in quadrature with the uncertainty of the reference value of Hp(10). Monte Carlo uncertainties are 1%. Source

0

20

rH (cm2 241

Am-Be Cf 252 Cf(D2O) 252

rH (cm2

mSv1)x 106

RH (cm2 mSv1)  103

0.863 1.19 11.6

1.23  0.09 1.61  0.08 16.6  1.0

40

rH (cm2

mSv1)  106

RH (cm2 mSv1)  103

0.833 1.13 11.1

1.16  0.11 1.53  0.08 15.6  0.8

241

Am-Be; 4%–5% for the 252Cf and 3%–6% for the 252Cf(D2O)) are the standard deviations of the reading of the nine dosemeters summed in quadrature with the uncertainty of the reference value of Hp(10). In the hypothesis that the simulation and the experiments are coherent, the ratio rH/RH should remain constant over all irradiation scenarios. From Table 3 the ratio rH/RH appears constant, within the uncertainties, for all irradiation tests. As an estimation of the degree of coherence between simulation and experiment, the standard deviation of all data in Table 3 may be taken. The result (5%), fully coherent with the experimental uncertainties, qualifies the model as accurate enough for the requirements of the dosemeter design. The average of the numbers reported in Table 3 represents the best estimation of the ‘‘scaling factor’’ for which the result of the simulation should be divided to obtain the dosemeter reading (track density per unit Hp(10)) in a generic irradiation scenario. Its value is 714  10. The large value of this scaling factor comes from the following effects. Firstly, the alpha particles emerging from the thermal neutron capture in 10B mainly have energy 1.47 MeV and their range in 10B is about 6 mm. Considering the large thickness of the converter (50 mm), the high percentage of 10 B and the high capture cross section of thermal neutrons in 10B (about 3800 barn), it can be estimated that the alpha particles produced in the few mm layer facing the PADC are about 1% of the total number of alphas produced in the whole converter thickness. Moreover, it should be considered that only the alpha particles reaching the PADC with incidence angle lower than a critical value are registered with efficiency larger than 50%. Supposing 45 as critical angle (Calamosca et al., 2003), only one out of about 7 alphas produced in the mentioned layer will be registered, resulting in an overall normalization factor of about 700. 5. Evaluation of the angle dependence of the dosemeter response As anticipated in Section 1, one of the main advantages of the albedo dosemeter is the limited dependence of the response on the irradiation angle. A recent ISO Standard (ISO, 2005) prescribes, as a performance requirement for the angular response of albedo neutron dosemeters, that the arithmetic mean of the response at angles ranging from 0 to 60 shall not differ by more than 30% from the corresponding response at normal incidence. By applying this criterion to the experimental data, the required arithmetic

Table 3 Ratio between the simulated number of (n,a) reactions in the boron screen and the measured track density in PADC for different calibration fields and angles of incidence.

rH/RH (102) Angle

241

252

0 20 40 60

7.0  0.5 7.2  0.7 7.0  0.8 6.7  0.6

7.4  0.4 7.4  0.4 7.1  0.3 7.6  0.3

Am-Be

Cf

252

Cf(D2O)

7.0  0.4 7.1  0.4 6.5  0.3 7.7  0.3

60

rH (cm2

mSv1)  106

RH (cm2 mSv1)  103

mSv1)  106

RH (cm2 mSv1)  103

0.729 0.993 9.46

1.04  0.12 1.40  0.05 14.5  0.7

0.601 0.816 7.33

0.90  0.08 1.08  0.05 9.5  0.3

mean normalized to the normal incidence data is 0.88, 0.87 and 0.85 for the 241Am-Be, 252Cf and 252Cf(D2O), respectively. This fully satisfies the ISO criterion. 6. Evaluation of the detection limit An important feature of a dosemeter is the value of the detection limit, DL, i.e. the dose level at which a known fraction (5% is considered here) of type II errors (false negatives) may occur. This can be evaluated with the formula proposed by Tanner and Bartlett (1995):

DLw2$ k$ðsB þ 1Þ=S

(1)

Where sB is the standard deviation of the reading of non irradiated dosemeters and S is the dosemeter sensitivity (cm2 mSv1). The coverage factor k is 1.65 for 95% confidence level. Since the dosemeter response decreases with the neutron energy, the sensitivity S was conservatively chosen for 241Am-Be neutrons. Thirty unirradiated dosemeters were used to derive sB, obtaining 15 cm2. Using S ¼ 1230 cm2 mSv1 (see Am-Be at 0 in Table 2), the result is DL w 40 mSv. Since this albedo configuration will be used to assess the personal dose equivalent associated to the low energy component of workplace neutron fields (<100 keV), the expected operative detection limit will be significantly enhanced. 7. Conclusions An albedo neutron dosemeter based on a planar PADC covered with a 10B converter was designed. Its response in terms of personal dose equivalent Hp(10), as a function the energy and angle of incidence, was investigated through experiments in reference radioisotope neutron fields and simulations with MCNP-4C. The agreement between measurements and simulations is satisfactory, allowing predicting the dosemeter reading in generic irradiation scenarios. The angle dependence of the dosemeter response, evaluated through recent ISO recommendations, is satisfactory. Moreover, preliminary studies of the background distribution showed that the achievable detection limit should be lower than 40 mSv in workplace neutron fields. As expected, this configuration has prominent energy dependence (a factor seven when changing the energy from thermal up to 1 MeV neutrons), which could be partially corrected by introducing a Cd filter to attenuate thermal neutrons coming from the front and from the sides. In that case, the absolute response would decrease and the detection limit in the thermal – epithermal region would increase. For high-energy sources as the Am-Be no changes are expected, because the source spectrum has no thermal neutrons and the results of the irradiation tests were corrected for the room- or air-scattered neutrons. Taking advantage of the studied albedo configuration and of a PADC directly exposed to the neutron field, a combined dosemeter will be assembled with the aim of providing a single dosemeter with satisfactory response in a broad energy range (from thermal up to

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20 MeV neutrons). Further experiments are foreseen to investigate the effect of a Cd filter to improve the energy dependence of the response in the thermal and epithermal domains. Advantageous characteristics will be the insensitivity to photons and the possibility to process the low energy (PADC albedo) and the high-energy (directly exposed PADC) sections with the same equipment, in terms of chemical laboratory and track reader. These should constitute a remarkable advantage with respect to the existing combined dosemeters (TLD albedo þ directly exposed PADC), that need a TDL reader in addition to the PADC processing equipment. References Bedogni, R., Gualdrini, G., Monteventi, F., 2002. Field parameters and dosimetric characteristics of a fast neutron calibration facility: experimental and Monte Carlo evaluations. Nucl. Instrum. Methods A 476, 381–385. Bedogni, R., Esposito, A., Lovestam, G., Garcı´a, M.J., Virgolici, M., 2008. The new PADC based fast neutron dosimetry system of the INFN-LNF. Radiat. Meas. 43, S491–S494. Bordy, J.M., Stadtmann, H., Ambrosi, P., Bartlett, D.T., Christensen, P., Colgan, T., Hyvo¨nen, H., 2001. EURADOS trial performance test for neutron personal dosimetry. Radiat. Prot. Dosimetry 96 (1–3), 167–173. Version 4C LANL Manual LA-13709-M Los Alamos. In: Briesmeister, J.F. (Ed.), MCNPdA General Monte Carlo N-Particle Transport Code. Los Alamos National Laboratory), Los Alamos, NM.

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