Experimental gas amplification study in boron-lined proportional counters for neutron detection

Radiation Measurements 42 (2007) 245 – 250 www.elsevier.com/locate/radmeas

Experimental gas amplification study in boron-lined proportional counters for neutron detection Dahmane Mazed ∗,1 Dipartimento di Ingegneria Meccanica, Nucleare e Della Produzione (DIMNP), Università di Pisa, Via Diotisalvi 2, I-56126 Pisa, Italy Received 8 February 2006; accepted 21 July 2006

Abstract Based on theoretical considerations related to the microscopic behaviour of electrons in gases in conjunction with a macroscopic hypothesis, a semi-microscopic gas gain formula for conventional proportional counters has been already developed and was afterwards successfully tested on gas gain data obtained in Ar–CH4 - and He–isobutane-filled proportional counters, using an X-ray radiation source. In the present work, it is verified that the semi-microscopic formula works as well in case of the detection of spread energy-spectrum of ionizing charged particles, such as involved within boron lined proportional counters, commonly used for thermal neutron detection. Indeed, we investigate experimentally the validity of the formula for the parameterization of the gas amplification factor data obtained in two boron lined proportional counters, filled with Ar–CH4 (10%) and Ar–CO2 (5%). The obtained results show clearly the ability of the gas gain formula to perfectly describe gas gain characteristics in such counters independently of the filling gas mixture used. Consequently, this provides further evidence of generality of the underlying gas amplification semi-microscopic model. © 2006 Elsevier Ltd. All rights reserved. Keywords: Proportional counters; Gas gain formula; Semi-microscopic formula; Gas amplification; Neutron detection; Boron

1. Introduction Boron-lined proportional counters are usually employed for thermal neutron detection. Generally, they are the mostly used due to their good discrimination property against gamma background and also for their long life duration without suffering from any deterioration of their counting and spectrometric characteristics such as that observed in boron trifluoride (BF3 ) proportional counters, when operating in a relatively high ambient temperature and/or in the presence of an unavoidable gamma flux component mixed to the neutron field such as existing within and around nuclear reactor facilities (Baaliouamer et al., 1999). The applications of boron-lined counters include thus reactor control instrumentation, delayed neutron monitoring ∗ Tel.: +39 050 836649.

E-mail address: [email protected]. 1 Formerly: Commissariat à l’Energie Atomique, Centre de Recherche

Nucléaire de Birine, B.P. no. 180, Aı¨n-Ousséra, 17200, Wilaya de Djelfa, Algeria. 1350-4487/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2006.07.009

(for fuel failure detection) in nuclear reactors and nondestructive assay of spent fuel elements, etc. (Mazed, 2001). It would therefore be advantageous to improve the neutron sensitivity of boron-lined counters to the level of BF3 or 3 He counters. This is achieved by selecting optimum coating thickness (0.4.0.8 mg/cm2 ) and by increasing the coated area (Dighe et al., 2003). It is well known that high gamma fluences induce an excessive increase of dissociation rate of the BF3 gas molecule, resulting in a so dramatic increase of fluorine free radicals (F, F2 , F3 ) concentration within BF3 counter (Dauphin and Duchene, 1983). These radicals, being so highly electronegative, attach easily a more and more important fraction of those liberated free electrons in the gas, before they could reach the collecting anode wire. Therefore, boron coated proportional counters would be capable of operation in gamma background that is considered too high for BF3 by setting a suitable operating gas amplification factor value. Consequently, to improve their design, one requires a good knowledge of the dependence of their amplification characteristics on the filling gas mixture being used, the pressure and

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D. Mazed / Radiation Measurements 42 (2007) 245 – 250

Table 1 General specifications of the boron-lined proportional counters used Specifications

DIC 1 no 3

LND 232

Maximum diameter (cm) Internal diameter (cm) Cathode material Total length (cm) Effective length (cm) Anode diameter (nature) Neutron sensitive material 10 B enrichment Coated thickness (mg/cm2 ) Coated area (cm2 ) Insulation material Electrical capacitance (pF) Gas gilling Gas pressure (Torr) Thermal neutron sensitivity (cps/nv)

2.54 2.3 SS 18/8 25 10 50 m (W) Boron (10 B) 90% 0.5 77 Al2 O3 + PTFE 4.6 Ar–CH4 (10%) 200 1.2

2.55 2.36 SS 19.4 11.13 25 m (W) Boron (10 B) (96%) 0.5 33 Al2 O3 4 Ar–CO2 (5%) 200 1.0

other electrical and geometry parameters. A theoretically well founded semi-microscopic gas gain formula for proportional counters was developed in the past (Mazed and Baaliouamer, 1999). Since it was so far only tested for very short radiation ionization tracks (of the mm order) such as X-rays, it is felt the need to check whether the physical basis of the model still holds for charged particles, such as alpha particles, characterized by spatially extended ionization tracks within the gaseous sensitive volume of proportional counters. To this connection, it should be worthwhile to examine further the effect of a large spread distribution of ionization tracks on the applicability of the semimicroscopic gas gain formula. For this purpose, we choose to test experimentally the validity of this formula in case of boron-lined proportional counters commonly used for thermal neutron detection in several applications. This is achieved by performing experimental gas gain measurements in two similar geometry boron-lined proportional counters. The first boron-lined proportional counter (LND 232), being a commercial model available in the market (LND Inc.), is filled up to 200 Torr with an Ar–CO2 (5%) gas mixture, whereas the second one (DIC 1 no 3), designed with a similar geometry, is a prototype model developed at the Detection & Measurement Laboratory of the Centre de Recherche Nucléaire de Birine (CRNB) and is filled to the same pressure with an Ar–CH4 (10%) gas mixture. The general specifications and mechanical characteristics of these counters are summarized in Table 1. It is the scope of this paper to briefly present the main results achieved in this work and also to discuss the extension of the applicability of the semi-microscopic formula to describe gas gain data obtained using ionizing charged particles, characterized by a large spread of extended ionization tracks, such as the case in boron-lined proportional counters. 2. The semi-microscopic gas gain formula Starting from the microscopic definition of the first Townsend coefficient and using a set of theoretical considerations related to the physical behaviour of electrons in gases together with

an empirical macroscopic hypothesis, an analytical formula of /P , the first Townsend coefficient to pressure ratio, has been developed in the form (Mazed and Baaliouamer, 1999)     S0 1+m  = CS exp − , (1) P S where  stands for the first Townsend coefficient, S = E/P is the electric field strength to pressure ratio, whereas C, S0 , are constants depending on the nature of the gas, m is a parameter ranging between zero and unity (i.e., 0 m 1), depending on the complex shape of electronic cross sections of the gases involved. Substitute this functional dependence of /P in the gas gain equation given by  Sa Ln A  dS = , (2) P · a · Sa P S2 Sc where A is the gas gain, Sa and Sc stand for the electric field strengths to pressure ratio at the anode wire surface of radius a, and at the starting point of the gas amplification process near the central wire respectively. After integration, we obtain the general semi-microscopic gas gain formula      Ln A 1 S0 1+m Sa + = K (1 + m) Ln P · a · Sa S0 1.1! Sa   2(1+m)   1 S0 1 S0 3(1+m) − + − ··· 2.2! Sa 3.3! Sa − L,

(3)

where S0 , K=C/(1+m) and L are constants determined only by the specific nature of the gas, and m is a moderation parameter valued in the range of 0 m 1. The exact physical significance of these gas constants was already extensively discussed in previous contributions (Mazed and Baaliouamer, 1999; Mazed et al., 2001; Mazed, 2002; Mazed and Amokrane, 2002). When plotting Y = Ln A/P · a · Sa against the expression in braces, which is taken as the control variable (hereafter, denoted by X), we shall obtain a straight line if the semi-empirical model underlying this formula really holds. The general formula stated above shows that when restricting the expansion of the control variable to the zero order term, i.e., keeping only the natural logarithm term, one obtains a general form of the well-known Diethorn formula with m in the range of 0 m1. When m is zero, this simply reduces to the original Diethorn (1956) formula. When extending the expansion to the first order term approximation, we keep the two first terms of the control variable, and we obtain the generalized Zastawny formula for m between zero and unity. In the particular case of m=0, this first order approximation reduces simply to the classical Zastawny formula (Zastawny, 1966). Although these two approximations are sometimes found sufficient to describe roughly the experimental gas gain data (Baaliouamer et al., 1996), the second and third order approximations are, however, often required when accurate parameterization is desired over an extended range of working variables, particularly in the low Sa range (Curzio et al., 2005). In the third order approximation, one has to keep

D. Mazed / Radiation Measurements 42 (2007) 245 – 250

247

Cadmium sheets Counter cathode Anode

High Voltage Electrometer

Pu-Be neutron Source

Boron-lined proportional counter

Paraffin wax

Fig. 1. The experimental setup of gas gain measurements using the current method. In order to ovoid the end-effect, the counter ends were screened by a cadmium sheet.

3. Experimental setup The experimental setup is shown in Fig. 1. The neutron flux is provided from a 2 Ci activity Pu–Be isotopic neutron source, thermalized within a paraffin wax-filled container. This installation can provide thermal flux densities up to 2×103 cm−2 s−1 with negligible gamma dose rate component. The gas gain values were measured using the current method, as described in a previous paper (Baaliouamer et al., 1996). This method is usually preferred to the pulse matching technique which requires a great deal of care regarding the correction of the ballistic deficit due to the amplifier time constants shaping networks (Hendricks, 1973; Baaliouamer et al., 1994). However, we shall note that the current method has to be conducted carefully, avoiding the systematic errors that could arise from a possible mixing of electron avalanche chains (secondary avalanches) of low gas gain occurring at the ends of the anode wire (end effects). These drawbacks can be overcome, by screening the counter ends with a cadmium sheet, restricting thus the neutron irradiation to a central zone (6 cm long) of uniform gas gain region, that is close to the central part of the counter sensitive length (refer to Fig. 1). Indeed, in that region the gas amplification is quite uniform, as can be seen in Fig. 2, showing the gas gain lineal profile obtained along the sensitive length of the counter for a given high voltage value (750 V). For the gas gain measurements, performed at room temperature (296 K), the counters were polarized using a negative high voltage power supply (Tennelec TC932) with a resolution of 0.5 V on the cathode and the ionization currents collected at the anode wire were monitored by a programmable electrometer (Keithley 617) sensitive to currents of about 10−16 A, this permitted us therefore to measure gas gain values with an accuracy better than 4%.

1.2 Relative gas gain : A / Amean

four terms of the control variable development, i.e.,      Sa 1 S0 1+m X = (1 + m) Ln + S0 1.1! Sa   2(1+m)   1 S0 1 S0 3(1+m) . − + 2.2! Sa 3.3! Sa

HV = 750 Volts Amean = 106

1.0

DIC 1'' n°3

0.8 uniform gas gain region (l = 6 cm)

0.6 0.4 sensitive length middle

0.2 0.0 -6

-5

-4

-3 -2 -1 0 1 2 3 Sensitive length position [ cm ]

4

5

6

Fig. 2. Pulse height distribution spectra of the boron-lined proportional counters used for gas gain measurements. The thermal neutron flux at the irradiation channel was about 2 × 103 n/cm2 s.

The high voltage value was varied from 0 up to 1 kV and the monitored current intensities ranged between 2 × 10−13 and 4 × 10−10 A approximately.

4. Pulse height distribution in boron-lined proportional counters Boron-lined proportional counters are used to detect thermal neutrons through the detection of one of the two recoil particles,  and 7 Li, which are produced by the neutron nuclear reaction 10 B(n, )7 Li, having a cross section of about 3840 barns for 2200 m/s neutrons, taking place when an incident thermal neutron interacts with a 10 B atom deposited on the inner cathode wall of the counter, following the nuclear reaction

(4)

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D. Mazed / Radiation Measurements 42 (2007) 245 – 250

3x104 DIC 1'' ( n°3 ) ( gas gain = 106 )

Count [arb. units]

LND232, ( gas gain = 180 )

2x104

1x104

Φnth = 2. 103 n.cm-2s-1 (Thermal neutron flux in the channel )

0 0

100

200

300 400 500 MCA Channel Number

600

700

Fig. 3. The gas amplification lineal profile along the sensitive length of DIC 1 no 3 counter at a high voltage value of 750 V. The mean gas gain is about A = 106. The deduced length of uniform gas amplification region is about 6 cm centred on the middle of the counter sensitive length.

Since  and 7 Li are emitted at 180◦ of each other, only one of them emerges from the boron substrate and penetrates into the sensitive gas mixture of the counter. Therein, the emerging electrically charged particle releases all or a given amount of its remaining kinetic energy, depending whether it is stopped within the sensitive gas or encounters the counter wall cathode (wall effect). The gamma photon emitted by the decay of 7 Li∗ in the most probable mode has an energy of 480 keV. The energy and momentum conservation laws predict kinetic energies of E = 1.47 MeV and ELi = 0.84 MeV in the most probable mode (93.7%), as shown in reaction (4), and, respectively, 1.775 and 1.017 MeV in the other mode. Natural boron contains about 18.8% (by mass) of 10 B, so in order to enhance the thermal neutron sensitivity, 10 B enriched boron (up to 95%) is used industrially for the manufacture of such counters. Since the average ranges of the recoil particles in solid boron 90% 10 B enriched (mass density = 2.195 g/cm 3 ), as estimated using SRIM program (Ziegler, 2004), are 0.88 and 0.47 mg/cm2 , respectively, for  (E¯  = 1.49 MeV) and 7 Li (E¯ Li = 0.85 MeV), therefore optimum thickness of the solid converter layer should be within the range of [0.4.0.8 mg/cm2 ]. Owing to the energy loss of these particles in the boron coating, the emerging kinetic energies of the particles are then continuously distributed from 0 to E¯  =1.49 MeV for  particles and from 0 to E¯ Li =0.85 MeV for 7 Li particles. The continuous energy spectrum results thus in two flat plateaus delimited from the high energy side by an abrupt edge corresponding to - and 7 Li-particles, in the delivered pulse height spectrum, as can be seen in Fig. 3. These pulse height distributions were recorded using an electronic chain constituted by a low-noise charge sensitive preamplifier (Canberra 2006 E), a research pulse amplifier (EG&G ORTEC 450) and a multichannel pulse height analyser (MCA, Canberra S 35 plus). The DIC 1 no 3 and LND 232 boron-lined proportional counters were polarized at high voltage values of 750

and 650 V, respectively, and the pulse shaping time constant of the pulse amplifier was set to 2 s. 5. Results and discussion Prior to commencing the experimental gas gain measurements, and in order to avoid systematic errors arising from the low gas gain values occurring at the ends of the counters, we have first, measured the gas gain lineal profile. This was done by manufacturing a 30 cm long cadmium tube containing a slit at its middle. The boron-lined proportional counters were therefore inserted inside during current intensity measurements. A total of 25 measurement points were taken when moving the slit along the whole sensitive length of each counter. The constant value of high voltage was set at 600 and 750 V, respectively, for LND232 counter and DIC 1 no 3 counter. As we can see in Fig. 2, showing the lineal gas gain profile, the uniform gas amplification region is located 3 cm on either side of the middle of the counter sensitive length. So, in subsequent gas gain measurements, only this part (6 cm long) of the counter is irradiated with thermal neutrons. This insures us to deal with a uniform gas gain process along the anode wire with an accuracy of atleast 5%. Fig. 4 shows the experimental gas gain data (in symbols) obtained for the Ar–CH4 (10%) and Ar–CO2 (5%), together with the recalculated amplification characteristics (in solid lines) using the gas gain constants extracted from least squares fitting analyses of the corresponding experimental data. The gas constants used are those summarized in Table 2. We see that the gas gain evolves more rapidly with high voltage value in LND 232 counter than it does in DIC 1 no 3 counter. This is expected since the amplification process is more sensitive than the quenching gas fraction admixed, 5% of CO2 in the former and 10% of CH4 in the latter. For example, while a gain value of

D. Mazed / Radiation Measurements 42 (2007) 245 – 250

249

Table 2 Summary of the gas constants extracted from mean square fittings of the semi-microscopic gas gain formula (m = 2/3) taken in its third order approximation to the experimental gas gain data obtained in Ar–CH4 (10%) and in Ar–CO2 (5%), using boron-lined proportional counters Counter

K (10−2 V−1 )

S0 (V/cm Torr)

L (10−3 V−1 )

Correlation coefficient

Gas gain range validity

DIC 1 no 3 LND 232

1.44 ± 0.07 1.45 ± 0.07

35.0 ± 1.8 22.0 ± 1.1

7.3 ± 0.1 3.5 ± 0.1

0.9994 0.9992

1.1–1000 1.1–3000

0.05

104

zero order ( r = 0.9945 )

Ar-CH4 ( 10 % ) ( DIC 1'' n°3 )

1st order ( r = 0.9973 )

0.04

Fitted curves

ln A / P.a.Sa [ Volt -1]

103 Gas Gain, A

Ar-CO2 ( 5 % ) ( LND 232 )

102

101

2nd order ( r = 0.9990 ) 3rd order ( r = 0.9994 ) Best linear fitting

0.03

0.02

Gas : Ar-CH4(10 %) Deduced Gas constants : So= 35 V/cm.torr m = 0.666 -2 -1 K = 1.44 10 V L = 7.33 10-3 V-1

0.01 100 0

200

400 600 High Voltage [ Volts ]

800

1000

Fig. 4. Experimental gas gain data (symbols) obtained in boron lined proportional counters used and the amplification characteristics fitted (solid lines) using the third order approximation of the general formula. Related gas constants values are given in Table 2.

100 is achieved in LND 232 counter when setting a high voltage of 600 V, in DIC 1 no 3, the gas gain value is only 25 at the set voltage value. One has to increase the voltage by about 150 V to get the same gas amplification level, usually required in many applications. On the other hand, in order to show the effect of extending the control variable to higher orders, we plot in Fig. 5 the successive approximations of the fitting formula, namely: the zero order, the first order, the second and the third order approximations, when keeping the parameter m (=0.666) constant. We can notice clearly the progressive achievement of a better fit of the experimental gas gain data obtained in DIC 1 no 3. The corresponding improvement of the correlation coefficient value (very close to unity) of the successive approximations is indicated in the upper frame inserted in Fig. 5. This fact is also obtained when dealing with gas gain data from LND 232. Therefore, this permits us to conclude that the microscopic gas gain formula describes rather well the gas amplification mechanism also in boron-lined proportional counters, whatever be the nature and the fraction of the admixed quenching gas being used. Indeed, in the theoretical development of the model (Mazed and Baaliouamer, 1999), as a working hypothesis, it was assumed to deal with proportional counters filled with a binary gas mixtures of which the main gas should be a rare gas, such as argon, whereas the quenching gas should be a molecular gas such as CO2 , CH4 and so on.

CP DIC 1'' n°3

0.00 0

1

2 Control Variable, X

3

4

Fig. 5. Effect of the successive extension of the expansion of the control variable up to the third order term in DIC 1 no 3 counter filled up to 200 Torr with Ar–CH4 (10%) gas mixture. The obtained improvements of the correlation coefficient are indicated between parenthesis in the figure legend (upper frame).

Although the measurements described here involved extended ionisation tracks with a large spread of energies, no significant differences were noticed for the gas amplification mechanism compared to earlier measurements (Mazed and Baaliouamer, 1999). And this was also pointed out in our recent work (Curzio et al., 2005), when we were dealing with the air effect on gas amplification characteristics in proportional counters intended for airborne radon monitoring. A perfect proportionality operation can also be noticed on the basis of the pulse height spectra evolution. For example, a perusal of the pulse height spectra depicted in Fig. 3, shows that the ratio between the maximum energy value of the alpha and 7 Li energy distributions, i.e., 1.49 MeV and 0.85 MeV, is quiet equal to the ratio between the channel numbers of the corresponding edges (350 and 200, respectively). 6. Conclusion The results presented in this paper lead us to conclude that the semi-microscopic gas gain formula developed originally for gas gain parameterization in conventional proportional counters applies well also to other types of proportional counters such as those described here. This result was quite expected. Indeed, the major difference between boron lined proportional counters and the conventional ones consists only in the converter material

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inserted in the counter, on which thermal neutrons react. Hence, providing appropriate values of the gas constants, the semimicroscopic gas gain formula is expected to describe acceptably the gas gain in boron-lined proportional counters, whatever be the nature of the filling binary gas mixture. Finally, as a future work, we plan to check the validity of this formula for gas gain description in boron trifluoride proportional counters. In such counters indeed, there is no rare gas; these are filled only by BF3 , a molecular gas. Acknowledgements The author is thankful to Mr. M. Baaliouamer, Director of Prospecting at The Algerian Atomic Energy Commission (COMENA) for his constant encouragement during the course of our investigation. Thanks are also due to Mr. S. Mameri and Miss K. Negara of the CRNB Nuclear Research Center for their grateful contribution in the course of performing the experimental measurements. References Baaliouamer, M., Belaragueb, C., Mazed, D., 1994. La technique impulsionnelle pour la caractérisation des détecteurs à gaz. CEA Report, CEA-N-2756, p. 47. Baaliouamer, M., Belaragueb, C., Mazed, D., 1996. Gas gain measurement in helium–isobutane mixtures filled-proportional counters. Nucl. Instrum. Methods A 382, 490–494. Baaliouamer, M., Mazed, D., Mameri, S., Negara, K., Belaragueb, C., Zitouni, H., 1999. Neutron detectors for research reactor control. Communication Presented at the First AFRA/IAEA Conference on Research Reactor Operation, Safety and Utilization, Algiers, Algeria, 10–11 April 1999.

Curzio, G., Mazed, D., Ciolini, R., Del Gratta, A., Gentili, R., 2005. Effect of air on gas amplification characteristics in argon–propane (1%)-based proportional counters. Nucl. Instrum. Methods A 537, 672–682. Dauphin, G., Duchene, J., 1983. Compteurs proportionnels à dépôt de bore, à réponse rapide et longue durée de vie, pour les ensembles de démarrage des réacteurs. CEA Report, Note CEA-N-2299. Diethorn, W., 1956. A methane proportional counter system for natural radiocarbon measurements. US AEC Report, NYO. 6628. Dighe, P.M., Prasad, D.N., Prasad, K.R., Kataria, S.K., Athavale, S.N., Pappachan, A.L., Grover, A.K., 2003. Boron-lined proportional counters with improved neutron sensitivity. Nucl. Instrum. Methods A 496, 154–161. Hendricks, R.W., 1973. A pulse-matching method for estimating the gas amplification factor in proportional counters. Nucl. Instrum. Methods 106, 156–579. Mazed, D., 2001. Construction de détecteurs nucléaires en Algérie: opportunités présentes et perspectives futures. Communication Presented at the Fifth International Meeting of Studies on Marine Sciences, J’NESMA’2001, 28–30 May 2001, Algiers, Algeria. Mazed, D., 2002. Contribution to the study of the charge amplification in proportional counters. Proposition of a new semi-microscopic model for the prediction of the gas amplification factor. Magister Thesis, no. 13/2002/MPH, University of Sciences and Technology HouariBoumediène, Algiers, February 2002, unpublished. Mazed, D., Amokrane, A., 2002. Nouvelle formulation du facteur de multiplication des electrons dans les compteurs proportionnels. Bull. Bur. Nat. Metrol. BNM (Fr.), 123, 162. Mazed, D., Baaliouamer, M., 1999. A semi-microscopic derivation of gas gain formula for proportional counters. Nucl. Instrum. Methods A 437, 381–392. Mazed, D., Amokrane, A., Baaliouamer, M., 2001. Optimisation de compteurs proportionnels à base de l’argon et de l’hélium. Alg. Rev. Nucl. Sci. ARNS 3 (1,2), 85–97. Zastawny, A., 1966. Gas amplification in a proportional counter with carbon dioxide. J. Sci. Instrum. 43, 179–181. Ziegler, J.F., 2004. SRIM-2003. Nucl. Instrum. Methods B 219–220, 1027–1036.