The time-of-flight method for characterizing the neutron response of liquid organic scintillators

The time-of-flight method for characterizing the neutron response of liquid organic scintillators

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The time-of-flight method for characterizing the neutron response of liquid organic scintillators J. Iwanowska a, L. Swide a, T. Krakowski a, M. Moszynski a, T. Szczesniak a, G. Paus b a b

National Cenrskichtre for Nuclear Research, 05-400 Otwock-Swierk, Poland OncoRay—National Center for Radiation Research in Oncology, Fetscherstr. 74, PF 41, 01307 Dresden, Germany

art ic l e i nf o

a b s t r a c t

Article history: Received 18 July 2013 Received in revised form 14 January 2015 Accepted 14 January 2015

The purpose of this work is to present a measurement method for determining the neutron responses of various liquid organic scintillators using a time-of-flight technique in conjunction with a D–T neutron generator. The method is based on fast-neutron scattering on protons in a liquid-scintillator medium and on the acquisition of the neutron response of the medium as a function of the proton-recoil energy. This method can be applied to all scintillators that utilize fast-neutron elastic scattering. & 2015 Published by Elsevier B.V.

Keywords: Liquid scintillators Time-of-flight Recoil protons Quenching

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Description of the experimental method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Photoelectron yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Time resolution of the system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. TOF measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Fast-neutron elastic scattering on light nuclei is the most common method of neutron detection. The underlying principle of this interaction is the transfer of the neutron’s kinetic energy to the light nucleus, which results in a nuclear recoil. Afterward, the recoiling nucleus loses energy in the detector. Because the maximum transfer of energy occurs when the mass of the target nucleus is comparable to the neutron mass [1], hydrogenous materials are the preferred media for such detectors. Hydrogen-containing scintillators are sensitive not only to fast neutrons but also to gamma rays. However, the shapes of the scintillation signals caused by excitations by neutrons and gamma rays may be different. Gamma-ray- and neutron-induced fluorescence pulses contain short (prompt) and long (delayed) decay components. Generally, the yield of the slow

1 2 3 3 3 3 4 6 6

component depends on the energy loss (dE/dx) of the exciting particle and is greater for heavier particles [2]. The short-range recoil protons produced in the scattering of neutrons on hydrogen produce more delayed light emission than do the longer-range electrons produced in gamma-ray interactions. This fact results in differences in the measured pulses produced in certain organic scintillators (liquids and some plastics), which allows for the discrimination of neutrons and gamma rays [3]. The most commonly used detectors that utilize fast-neutron scattering are liquid scintillators, such as NE213 and its derivatives BC501A and EJ301, produced by SaintGobain and Eljen Technology, respectively. The luminous efficiency of a scintillator (the conversion factor between the deposited energy and the light emitted by the scintillator) depends both on the type (alpha, beta, gamma, etc.) and the energy of the incident particle that interacts with the scintillator

http://dx.doi.org/10.1016/j.nima.2015.01.051 0168-9002/& 2015 Published by Elsevier B.V.

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medium [4,5]. A large reduction in the light yield for charged particles, such as alpha particles or protons, is observed for organic scintillators; this reduction is much larger than that observed for inorganic scintillators. The phenomenon of particle energy that is not converted into visible light (heat, lattice vibrations, etc.) is called quenching. The first experimental data together with a parameterization of the light output curve for NE213 liquid scintillator were reported in [6–8]. More recent measurements of the response function of NE213 obtained using 0.5–6 MeV neutrons from a 252Cf source and the time-of-flight (TOF) technique were presented in [9]. A detailed study of the light signal generated by monoenergetic neutrons in BC501A was performed by Pozzi et al. [10] using MCNPX-PoliMi simulations. Another study of BC501A was performed by Gohil et al. [11] who used Geant4 code to generate the light output distributions of monoenergetic neutrons. Both authors reported very good agreement between simulations and analytical calculations and between simulations and measurements of the neutron light output response of BC501A. The recoil-proton-response method, which is based on a TOF technique in conjunction with mono-isotopic sources, isotopic sources such as AmBe, and neutron facilities that produce monoenergetic neutron beams, among others, has been demonstrated in many papers, including [12] and [13]. Aspinall et al. [14] demonstrated the effectiveness of the TOF technique using LS-301 organic liquid scintillator and a Van de Graaff accelerator. Enqvist et al. [15] reported the first results for the neutron light output response functions and detector resolution functions for three EJ309 liquid scintillation detectors of different sizes. The neutron beam was generated using deuterons from a Van de Graaff accelerator impinging on an 27Al target. In this paper, we present a measurement method for acquiring the neutron responses of various liquid organic scintillators at various recoil proton energies using the TOF technique in conjunction with a D–T neutron generator. A recent study of a high-flashpoint 10B-loaded EJ309B5 liquid revealed a thermal neutron peak at approximately 100 keV originating from energy deposited by recoil electrons (100 keV electron equivalent; keVee) [16]; the energy of this peak is significantly higher than that for xylene-based liquids, such as BC523A, BC523A2, and EJ339A2 ([17];  60 keVee). The observed differences suggest that high-flashpoint liquid scintillators exhibit different calibration characteristics than those of NE213 and BC501A.

Fig. 1. The geometry of the experiment. A liquid scintillator serves as the START detector, and a BC408 plastic scintillator is the STOP detector. The angle θ initially determines the scattered neutron energy.

The slowing-down process of a deuteron in the target: The range of 80-keV deuterons in the titanium target is approximately 0.6 mm. Under the simplifying assumption that the deuteron loses a uniform fraction of its energy per unit path length until it comes to a complete stop, the average energy loss is half of the initial energy. This approximation yields an energy dispersion of 20 keV at an angle of ϕ¼901. b) The finite size of the START detector: Because of the finite size of the detector, we observe neutrons emitted from the target at angles that can differ from 901 by up to 7 0.051. This phenomenon introduces a spread in the incoming neutron energy of 0.1 keV. a)

A neutron with an energy of 14.09 (70.02) MeV scatters on a hydrogen nucleus (proton) in a liquid-scintillator medium. Afterward, the singly scattered neutron is detected in the STOP detector. The fixed angle between the centers of the START and STOP detectors (the angle θ; see Fig. 1) determines the energy of the scattered neutron, En0 , through the equation En0 ¼ En cos 2 θ:

ð1Þ

The scattered neutrons recorded in our experimental setup at a fixed scattering angle exhibit a finite energy spread. To decrease the energy dispersion, we can determine the TOF of the scattered neutron, thereby allowing us to calculate En0 more precisely: 2

2. Description of the experimental method The method described in this paper is based on a D–T generator and two organic scintillators. A cylindrical liquid scintillator – EJ301, EJ309, or EJ309B5, with dimensions of 5.08 cm  5.08 cm – is used as the START detector, whereas a BC408 cylindrical plastic scintillator with dimensions of 5.00 cm  4.00 cm is the STOP detector (see Fig. 1). The maximum neutron flux generated by the D–T generator is 3  108 n/s. Deuterium ions are accelerated toward a titanium target (200 mm thick), in which a mixture of 50% deuterium and 50% tritium is implanted. Deuterons hit the tritium, creating a neutron and an alpha particle. The target contains deuterium and tritium; therefore, neutrons from the (d,d) reaction are also emitted. In our experiment, the deuterons were accelerated to an energy of 80 keV, and the resulting neutron flux from the (d,t) reaction was 2  107 n/s, whereas that from the (d,d) reaction was approximately 106 n/s. The angle ϕ between the direction of the deuteron beam and the emitted neutron (see Fig. 1) is 901. The neutron energy varies with the angle ϕ—for 901, En ¼14.09 MeV. The uncertainty of the incoming neutron energy (ΔEn) arises from two main sources:

E n0 ¼

d mn 2ðToFÞ2

;

ð2Þ

where d is the distance between the centers of the two detectors and mn is the neutron mass. We assume that a neutron with an energy of 14.09 MeV is non-relativistic (v¼ 0.17c). The uncertainty of the scattered neutron energy (ΔEn0 ) is caused by the following: a) The dimensions of the START and STOP detectors: The distance that a scattered neutron travels between the detectors lies in a range of 1997.5–2002.5 cm. Therefore, the value of ΔEn0 lies in a range of 6–9% (depending on En0 ). b) The time resolution of the system: This value was measured directly and was found to be 1 ns (see Section 4, subsection b). Afterward, the recoil proton energy (Er) in the liquid scintillator can be calculated from the equation Er ¼ En En0 :

ð3Þ

By changing the position of the STOP detector, we can scan different scattering angles, which translate into different recoil energies.

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All organic scintillators contain carbon and hydrogen. However, collisions between incoming neutrons and the carbon nuclei generate very little light. In the extreme case, a head-on collision, a 14 MeV neutron can transfer 100% of its energy to a proton or 28.4% of its energy to a carbon nucleus. Pulses from collisions involving carbon are very small compared with those generated on hydrogen; therefore, scattering on carbon was not considered in this experiment. We also assumed that each neutron collided only once in the START detector. Additionally, the TOF technique eliminates all events arising from scattering on carbon only.

3. Experimental setup The TOF method was used to determine the energies of the scattered neutrons. For application of this method, the START signal was obtained from the liquid scintillator under investigation, whereas the STOP signal was obtained from the BC408 plastic scintillator. The START detector was placed at a distance of 50 (7 1) cm from the D–T generator. Both scintillators were coupled to Photonis XP20D0 fast photomultipliers (PMTs) to ensure excellent time resolution. The fast anode signals from the detectors were fed into a five-channel constant-fraction timing discriminator (CFD). The time difference between the START and STOP signals was measured using a time-to-amplitude converter (TAC). The logic signals from the outputs of the CFD were also fed into a coincidence module that created a gate to trigger a dataacquisition system. The data from the TOF experiment were collected using Kmax 7.4.4 [18]. Additionally, a pulse-shape discrimination (PSD) technique was applied to separate neutrons from gamma rays. The PSD was performed using a zero-crossing (ZC) method. The anode pulse from the liquid scintillator was fed into an NDE202 module [19], which was originally developed for the EUROBALL neutron wall [20]; it provides fast timing signals from the outputs of the CFD and the ZC discriminator and operates based on a bipolar-shaped signal. The CFD signal was further used as a start signal for the TAC module, whereas a ZC logical signal was used to stop the TAC. To obtain a pulse-height signal from the START detector, we fed the dynode signal into a preamplifier, which was connected to a shaping amplifier with a 0.5-μs time constant (Fig. 2). In the experiment, three parameters were acquired: – the pulse height, – the ZC time, and

3

– the time difference between the arrivals of the START and STOP signals, x (see Eq. (4)), from which the TOF can be calculated. Additionally, the photoelectron yields for all tested liquid scintillators were measured. The scintillators were coupled to the same PMT as in the TOF experiment (Photonis XP20D0, with a blue sensitivity of 13.2 μA/lmF). The data were collected and analyzed using the Tukan8k acquisition system [21]. The numbers of photoelectrons were determined using the Bertolaccini method [22,23]. The number of photoelectrons was measured by comparing the position of the Compton edge for 662-keV gamma rays detected in the scintillator to the position of the single photoelectron peak (which determines the gain of the PMT).

4. Results 4.1. Photoelectron yield The numbers of photoelectrons per energy unit measured for all tested liquid scintillators are presented in Table 1. The photoelectron numbers measured using the XP20D0 are lower than those reported in [16] for the tested liquid scintillators; this difference can be attributed to the lower photoelectron collection efficiency from the external portions of the photocathode in the fast PMT. 4.2. Time resolution of the system The time calibration of the system, which was found to be 24.41 ps/channel, was performed in coincidence with 511-keV annihilation photons from a 22Na gamma-ray source. The spectra measured for 22Na and the time spectra of the coincidences in both detectors measured for the D–T generator are presented in Fig. 3. The time resolution of the system was approximately 1 ns. Table 1 Photoelectron yields of the tested liquid scintillators measured from the compton edge for 662-keV gamma rays. Scintillator

Photoelectron yield (phe/MeV)

EJ301 EJ309 EJ309B5

14007 70 13007 70 10007 50

Fig. 2. A diagram of the experimental setup.

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This value is crucial in determining the energy of a detected neutron. 4.3. TOF measurements

tube of the generator). The TOF of a scattered neutron is defined in terms of the following sum: ToF ¼ x þ t γ ;

ð4Þ

A representative plot of the ZC time as a function of the TOF measured for a 238PuBe neutron source is presented in Fig. 4. We observe four groups of coincidence events: gamma gamma (γ γ), neutron neutron (n n), neutron gamma (n γ), and gamma neutron (γ n) events. Neutrons from a 238PuBe source originate from the 9Be(α,n)12Cþ n nuclear reaction, where 238Pu is the α emitter. In this reaction, 4.4-MeV gamma rays are produced from the excited state of 12C*. 238Pu is also a spontaneous fission source, yielding, in our case, only 78 n/s (the mass of the 238Pu is 30 mg [24]), which is negligible with respect to the total number of n n coincidences (the total neutron flux from the 238PuBe source is approximately 5  105 n/s). The 240Pu composition in the 238PuBe source is not known, and therefore, there is likely to be a non-zero contribution to the neutron flux from other such isotopes as a result of spontaneous fission; however, it is assumed to be small. The next step was to use a D–T neutron generator and place the START detector 50 cm from the tube of the D–T generator. The angle between the detectors was fixed to 451. The results of this measurement are presented in Fig. 4. We observe γ γ and n–n coincidences concentrated in well-defined groups. The gamma rays in this case originate from background activation (predominantly in the metal

where x is the time difference between the arrivals of the START and STOP signals and tγ is the TOF of a gamma ray travelling between the two detectors. If the detectors are separated by a distance of 2 m, then tγ is 6.67 ns. The characterization measurements were performed using the D–T generator. We controlled the gain using the 137Cs, 60Co and 4.4-MeV gamma rays from the 238PuBe source, and we calibrated the system in electron equivalent energy units. The data were analyzed offline by applying narrow energy cuts (corresponding to 7100 keV in energy) on the TOF axis (to determine, with fairly high accuracy, the energy of the recoil protons) and the ZC axis (to reject the gamma-ray background). Afterward, the chosen data were projected onto the electron equivalent energy axis. Representative results of these projections for 4-MeV and 5-MeV recoil protons are presented in Fig. 5. The FWHM/E values of the spectra were 17% for 5-MeV recoil protons and 20% for 4-MeV recoil protons. The response functions of three tested scintillators, which represent the light output (L, in units of MeVee) versus the kinetic energy of the recoil protons (Ep), are displayed in Fig. 6 for an energy range of 0.5–11 MeV. The greatest shortcoming of the method used in the experiment is the relatively large uncertainties, particularly in the

Fig. 3. Timing spectra measured using the EJ301 liquid scintillator.

Fig. 5. The projection of the TOF cuts onto the electron equivalent energy axis.

Fig. 4. Two-dimensional plots of the zero-crossing (ZC) time vs. the TOF for the

238

PuBe neutron source (left) and for the D–T generator (right).

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5

Fig. 6. Light output (L) of the tested liquid scintillators as a function of the recoil proton energy (Ep).

low-energy region of the Ep axis. These uncertainties originate from the use of high-energy neutrons to measure low-energy recoil protons. These uncertainties could be reduced either by using a D–D generator, which produces lower-energy neutrons, or by increasing the distance between the detectors. However, a larger distance between the detectors would dramatically reduce the efficiency of the method. The data in Fig. 6 were fitted with several model functions using OriginPros software via a weighted least-squares method. Initially, the response functions were fitted using a linear empirical fit, similar to those presented in [25]. However, the curves thus obtained were nonphysical—for energies close to zero, the curves had negative amplitudes. The same situation occurred in the case of a quadratic fit, as proposed in [10]—the amplitude was either negative or greater than zero at zero proton energy. Another possible empirical formula, suggested by Kornilov et al. [26] and used in [15], is a ratio of polynomials: L¼a

Ep 2 ; Ep þb

Table 2 The model functions fitted to the measured data and their parameters for all tested liquid scintillators. Fit type

Polynomial (5) Exponential (6)

Coefficients EJ309

EJ301

EJ309B5

a¼ 1.89 7 0.12 b¼ 0.667 0.05 a¼ 0.7770.04 b¼ 2.63 7 0.17 c¼ 0.147 0.07 d ¼1.127 0.09

a¼ 1.94 7 0.08 b¼ 0.667 0.04 a¼ 0.737 0.09 b¼ 2.95 7 0.21 c¼ 0.117 0.07 d¼ 0.99 7 0.06

a¼ 1.29 7 0.11 b¼ 0.647 0.05 a¼ 0.78 70.07 b¼ 2.447 0.24 c¼ 0.157 0.08 d¼ 1.09 7 0.07

ð5Þ

where a and b are fitting parameters. An additional possible formula, presented by Cecil et al. [6] and also used in [15], is the exponential formula h  i L ¼ aEp b 1 exp cEp d ; ð6Þ where a, b, c, and d are fitting parameters. These fitting functions have the benefit of extrapolating to zero at zero proton energy without going negative for positive proton energies. Table 2 presents the parameters of fitting functions (5) and (6) for all tested liquid scintillators. The measured response curve for EJ309 and two corresponding fitting functions are presented in Fig. 7. The goodness of fit with respect to the detector type and fit type is represented by the value of the reduced chi-square parameter, which can be calculated from the equation χ 2reduced ¼

n χ2 1 X ðyi  y^ i Þ2 ¼ ; D Di¼1 σ2

ð7Þ

where χ 2 is the chi-square parameter, yi is a measured data point, y^ i is the corresponding fitted point, D is the number of degrees of freedom, and σ is the uncertainty of the data points. A reduced chi-square value near or equal to unity indicates that the extent of the agreement between the measured data points and the values predicted by the fitted curve is consistent with the error variance. A reduced chi-square value much larger than unity

Fig. 7. The measured light output curve for the EJ309 liquid scintillator together with the corresponding polynomial and exponential fitting functions.

indicates that the fit has not captured the data or that the errors have been underestimated. The reduced chi-square values estimated for the fitting functions for each scintillator are listed in Table 3. The goodness of fit indicated in Table 3 for the exponential fitting function is somewhat better than that for the polynomial function. However, the reduced chi-square values for both fitting functions are close to unity, with the exception of EJ309, for which the rational fit yields a higher value of this parameter (4.18). The response functions measured for each scintillator in the lower-energy range are displayed in Fig. 6. In the case of the highflashpoint media EJ309 (circles) and EJ309B5 (triangles), recoil protons create more light than they do in EJ301. This means that in

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neutrons, high-flashpoint liquid scintillators have lower neutron detection thresholds than do xylene-based scintillators. Therefore, EJ309 and EJ309B5 exhibit higher neutron detection efficiencies than does EJ301.

Table 3 Reduced chi-square values as indicators of the goodness of fit. Fit type

Scintillator

Polynomial Exponential

EJ309 4.18 1.15

EJ301 1.53 1.04

EJ309B5 1.52 0.99

Fig. 8. ZC time spectra measured for a 500-keV recoil proton energy for EJ301 (black line) and EJ309 (gray line).

high-flashpoint media, the quenching effect is lower than in xylene-based media. This result is in good agreement with the upshift in the thermal neutron peak observed in EJ309B5 in comparison with the xylene-based media BC523A and EJ339B5; see [16] and [17]. Furthermore, it suggests that EJ309 should offer superior PSD capability in the low-energy range for recoil protons because of its higher relative light output. The PSD quality can be quantified in terms of a figure of merit (FOM), which, for a Gaussian distribution, can be defined as follows: FOM ¼

Snγ ; FWHMγ þ FWHMn

ð8Þ

where Snγ is the separation of the gamma-ray and neutron peaks and FWHMγ and FWHMn are the full widths at half maximum of the gamma-ray and neutron peaks, respectively. The FOM calculated for EJ309 for a 500-keV recoil proton energy (see Fig. 8) is higher than that calculated for EJ301, which means that the PSD quality is higher in the case of EJ309. The PSD capabilities of EJ301 and EJ309 scintillators have previously been demonstrated by Stevanato et al. [27]; these authors compared EJ309 and EJ301 scintillators and showed that EJ309 exhibits inferior neutron/gamma discrimination properties. This finding was confirmed by Pawelczak et al. [28]. However, the two cited studies were both performed at the same gamma-ray energies. If we set the gates at the same recoil proton energy for both scintillators, we observe the opposite behavior—EJ309 has a higher FOM than that of EJ301 for proton energies below approximately 1.5 MeV. EJ309 produces a higher relative light output for low-energy neutrons than does EJ301—a 500-keV recoil proton energy gate corresponds to recoil electron energies of approximately 400 keVee in EJ309 and approximately 200 keVee in EJ301. As a consequence of their lower quenching for low-energy

5. Conclusions We presented a method for the neutron calibration of organic liquid scintillators based on a TOF technique, which can be applied to all scintillators that utilize fast neutron scattering. Using a D–T generator and relatively simple electronics, we obtained the light output response as a function of the recoil proton energy for three scintillators: EJ301, EJ309, and EJ309B5, each with dimensions of ∅ 5.08 cm  5.08 cm. The availability of a wide range of scattering angles allowed us to scan a large range of recoil proton energies. The measured data points spanned a range of recoil proton energies from 0.5 MeV up to 11 MeV. Several empirical fitting functions were compared with the data points; among these functions, the exponential curve exhibited the best agreement with the measured data. The primary disadvantage of the technique is the large uncertainties on the x axis, especially in the lowenergy region, which arise from the fact that we used high-energy incoming neutrons to measure low-energy recoil protons. Reducing these uncertainties by increasing the distance between the START and STOP detectors or by using smaller scintillators would reduce the efficiency of the method. The measurements indicated that high-flashpoint scintillators are characterized by lower quenching in the low-energy region than are low-flashpoint, xylene-based scintillators. As a consequence of this lower quenching in the low-energy region, EJ309 and EJ309B5 have lower neutron detection thresholds and higher neutron detection efficiencies than those of EJ301. References [1] G.F. Knoll, Radiation Detection and Measurement, fourth ed., Wiley, New York, 2010. [2] J.B. Birks, The Theory and Practice of Scintillation Counting, Pergamon Press, 1964. [3] N. Zaitseva, et al., Nuclear Instruments and Methods A 668 (2012) 88. [4] W.C. Kaiser, et al., IRE Transactions on Nuclear Science NS9 (1962) 22. [5] R.B. Murray, et al., Physical Review 122 (1961) 815. [6] R.A. Cecil, et al., Nuclear Instruments and Methods A 161 (1979) 439. [7] M. Anghinolfi, et al., Nuclear Instruments and Methods A 165 (1979) 217. [8] V.V. Verbinski, et al., Nuclear Instruments and Methods A 65 (1968) 8. [9] J.H. Lee, et al., Nuclear Instruments and Methods A 402 (1998) 147. [10] S.A. Pozzi, et al., Nuclear Instruments and Methods A 524 (2004) 92. [11] Gohil M. et al., in: Proceedings of the DAE Symp. on Nucl. Phys. vol. 56, 2011. [12] O.Y. Hu, et al., IEEE Transactions on Nuclear Science NS52 (2005) 473. [13] Zimbal A. et al. PoS (FNDA2006) 035. [14] M.D. Aspinall, et al., Nuclear Instruments and Methods A 583 (2007) 432. [15] A. Enqvist, et al., Nuclear Instruments and Methods A 715 (2013) 79. [16] L. Swiderski, et al., IEEE Transactions on Nuclear Science NS57 (2010) 375. [17] L. Swiderski, et al., IEEE Transactions on Nuclear Science NS55 (2008) 3710. [18] 〈http://www.sparrowcorp.com/old/kmax.html〉. [19] 〈https://nsg.physics.uu.se/sites/default/files/nde202_manual.pdf〉. [20] O. Skeppstedt, et al., Nuclear Instruments and Methods A 421 (1999) 531. [21] 〈http://www2.ipj.gov.pl/tukan_en/〉. [22] Bertolaccini M., et al. in: Proc. Nucl. Electr. Symp., Versailles, France, 1968. [23] M. Moszynski, et al., IEEE Transactions on Nuclear Science. NS44 (1997) 1052. [24] D. Reilly, et al., Passive Nondestructive Assay of Nuclear Materials, US Nuclear Regulatory Commission, 1991. [25] M. Marseguerra, et al., Progress in Nuclear Energy 43 (1–4) (2003) 305. [26] N.V. Kornilov, et al., Nuclear Instruments and Methods A 599 (2009) 226. [27] L. Stevanato, et al., Nuclear Instruments and Methods A 690 (2012) 96. [28] I.A. Pawelczak, et al., Nuclear Instruments and Methods A 711 (2013) 21.

Please cite this article as: J. Iwanowska, et al., Nuclear Instruments & Methods in Physics Research A (2015), http://dx.doi.org/10.1016/j. nima.2015.01.051i

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