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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 59, NO. 1, FEBRUARY 2012

Investigation of the Properties of 3 3 Different Scintillation Detectors for Neutron Activation Analysis Techniques M. Gierlik, J. Iwanowska, Member, IEEE, T. Kozłowski, M. Moszyński, Fellow, IEEE, L. Swiderski, Member, IEEE, and T. Szczesniak, Member, IEEE

Abstract—The quest for choosing the suitable detector for a specific homeland security application that takes an advantage of the neutron activation analysis technique prompted us to investigate cylindrical scintillation detectors. The properties of three emphasis was put on the detectors’ properties in the multi-MeV energy region and their response to the neutron radiation. In this work we compare the energy resolution, efficiency and timing properties of BGO, LaBr and NaI(Tl) scintillation detectors coupled to Photonis spectrometric photomultiplier tubes. Index Terms—Neutron activation analysis, scintillation detectors.

I. INTRODUCTION

T

HIS work was motivated by the research project aimed at designing a homeland security mobile system that takes an advantage of the neutron activation analysis technique (NAA) [1]–[3]. The NAA is a sensitive, analytical technique, which is used for qualitative as well as for quantitative analysis of numerous materials. The nuclei interacting with neutrons can be excited to the energies unavailable to any other, cheaply available method. Productive application of NAA requires the detector with sufficiently high detection efficiency for gamma rays up to the energy of 10 MeV [4], [5]. Detector size matters, when high detection efficiencies are in demand. However, having particular parameter fixed, we undertook an effort to compare the scintillation characteristics, such as the energy and timing resolutions, the stability and detection efficiency of different scintillators. In this work we investigated three scintillation materials, using commercially available cylindrical scintillators, each large, see Fig. 1 and Table I. LaBr was chosen because of its superior energy resolution, BGO is renown of its efficiency, while NaI(Tl) serves as the reference point for comparison. Plastic scintillators, even though extraordinarily fast and cheap, were excluded due to their poor efficiency and susceptibility to the neutron radiation. It Manuscript received February 25, 2011; revised June 28, 2011; accepted November 09, 2011. Date of publication January 05, 2012; date of current version February 10, 2012. This work was supported by EU Structural Funds, Project no. POIG.01.01.02-14-012/08-00. The authors are with the National Centre for Nuclear Research, PL 05-400 Świerk-Otwock, Poland (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNS.2011.2176350

Fig. 1. The photograph of the three tested detectors: NaI(Tl) (top), LaBr (right), BGO (left).

would be probably an interesting idea to take a closer look at BaF , unfortunately, there was no sample large enough in our possession. The authors believe that the results and conclusions presented in this paper can be helpful for users of scintillators with other shapes and sizes. The detection efficiency and the problems with continuous cooling have ruled out an HPGe detector. We know of at least one example of using the germanium detector in the NAA technique [6]. This option is extremely expensive and might be justified only by very unique requirements bestowed upon the device, such as the NIPPS ability to detect specific compounds used in the chemical weapons. Even though neutron fluxes emitted by the neutron generator are not sufficiently high to seriously damage the crystal structure, the requirement of neutron radiation hardness became one more argument to discourage us from acquiring an HPGE detector for our tests. As far as gamma radiation is concerned, the effective doses , received by any of our detectors are well below 500 and are by many orders of magnitude lower than potentially harmful doses, see [7]–[11]. Various physical properties of the crystals, collected for an easy reference in Table II, result in the different properties of complete detectors. Which one of them is supposed to be most suitable for the high energy gamma spectrometry, whether it is LaBr with its an excellent energy resolution or BGO with a

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GIERLIK et al.: INVESTIGATION OF THE PROPERTIES OF 3

3 DIFFERENT SCINTILLATION DETECTORS

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TABLE I APPLIED DETECTORS

TABLE II BASIC PROPERTIES OF THE STUDIED SCINTILLATORS Fig. 2. The energy resolution characteristics of BGO (solid circles), LaBr (solid triangles), and NaI(Tl) (open squares) detectors.

A. Energy Resolution

high peak-to-total fraction is the question that we try to answer in this work. II. EXPERIMENTAL RESULTS The scintillation materials and the associated measurement techniques in particular are well known to the reader, so there is no need to present them in details in this paper. However, we were making exceptions, in cases that we felt are not standard in laboratories, either because of rare combination of equipment, or because of its unorthodox usage. This is also the reason why we did not put too much of our interest in performance of our detectors in the energy region below 500 keV. For example, we completely disregarded a non-proportional response of scintillators, while, in the same time, had to deal with such phenomenon as an overloading, and in consequence, a nonlinearity of the photomultipliers. Latter aspect may be particularly interesting for LaBr users. In the work [15] authors describe how they disconnect the last dynode in their photomultiplier in order to decrease its gain. Such approach is feasible when accurate timing properties of the detector must be preserved. Otherwise, instead of modifying the voltage divider, it is far easier to decrease the bias voltage. In order to get the linear response up to the energy of about 20 MeV, the voltage in our lanthanum bromide detector with Photonis XP5700B PMT was reduced from the nominal 1 kV to 800 V. In a typical measurement the detector, consisting of the crystal, the PMT and the appropriate voltage divider provided a signal to a preamplifier and in turn to a spectroscopy amplifier such as Ortec 672 or Tennelec TC244. Signals were then processed by a PC -based Tukan multichannel analyzer [16], [17]. Power for the photomultipliers was supplied by Ortec 556 HV.

One would expect nothing new in investigating the energy resolution of well known scintillators. However, when it comes to large volume crystals, detection of MeV energy ranges, and collation of a number of less common detectors, a scrupulous explorer faces the tedious task of picking the desired information up, from numerous, scattered sources. In order to reach the energies as high as 10 MeV and to obtain a clear high energy peaks we took an advantage of our EADS Sodern Genie 16D neutron generator [18] to produce energetic gammas by means of fast neutron inelastic scattering or neutron capture of slowed down neutrons. The results can be seen in the Fig. 2. The Genie C/D neutron generators provide a controllable emission of a stabilized neutron flux in both continuous and pulsed modes. The model available to us was equipped with a deuterium-tritium neutron emission module (NEM), enabling the flux of a 14 MeV neutrons varying from n s up to n s, emitted almost isotropically at the full solid angle. A small, though measurable anisotropy can be observed in the neutrons energy and their distribution angle. The former is the result of D-T reaction kinematics, while the latter is caused by the generator steel casing deflecting and shielding the neutron flux along the NEM tube axis. In pulsed or burst modes the neutron pulse frequency can be, in principle, modulated between 10 Hz and 20 kHz. However, in real life, the design of Penning ion source sets some limitations on possible combinations of neutron pulse and cycle duration. This fact is well documented in the user manual. The manufacturer assures 4000 hours of an operation at n s. The 7.3 MeV and 10.2 MeV peaks are visible only in BGO crystals, as these are transitions in Ge. They are a result of the epithermal neutron capture by Ge, being one of the scintillator compounds (see Fig. 3). Finally, the exceptionally good energy resolution of LaBr allows identification of more subtle lines such as the transitions in after the neutron capture by stable chlorine isotopes in table salt. Access to higher energies is possible; however, it requires access to an accelerator to perform in-beam measurements [19]. The measured energy resolution (FWHM) data shown in Fig. 2 attract an eye; however, it is worth to note the real meaning of these nearly constant and rather low numbers. In fact, the peak width becomes larger with the increase of gamma

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Fig. 4. The figure illustrates the process of stabilizing detectors gain after turning them on. The temperature stability of crystals is probably one of the factors defining the stabilization time constants.

Fig. 3. BGO response to neutron radiation. 7.3 MeV and 10.2 MeV peaks are the result of epithermal neutron capture by Ge present in the scintillator. Other peaks come from nitrate fertilizer sample and surrounding of the detector. This BGO detector. particular spectrum was accumulated by the Cyberstar The germanium peaks were also observed in the investigated, smaller BGO scintillator, though they were considerably less outstanding.

energy. The energy resolution of 3% at 6 MeV means that any peak in this region has almost 200 keV at its half maximum, and that this number gets as large as 300 keV when the energy approaches 10 MeV. The effect of pair creation, adding the first and the second escape peaks, blurs the picture even more. From another point of view, worse energy resolution means that the same number of counts in a spectrum is smeared over a larger number of channels and so the signal to noise ratio is proportionally degraded, when compared to a scintillators with better energy resolution. The comprehensive discussion of the scintillators energy resolution issue is described in [20], which is a part of the textbook concerning medical applications of the scintillation detectors [21]. Unless the investigated gamma energy line is well isolated, as it is in the case of 10.8 MeV neutron capture line of , the task of distinguishing separate lines becomes extremely complicated, and/or weighted down with measurement and analysis uncertainties. B. Stability Almost every time a test system is moved away from its laboratory, the question about temperature stability arises. The best approach to solve this problem is to get the advantage of some temperature inert phenomena, compensate the changes or, if it is not the option, stabilize the environment, as in the case of the germanium detectors. In order to test the overall impact of environmental changes on the completely assembled detector and the associated electronics we measured the centroid position of the chosen peak (4.4 MeV) for about 18 hours. As we were interested in the system behavior right after it is turned on, the measurements were performed with electronics powered on just before the start of the measurement series (no warm up of the system). Known data regarding scintillators light output

temperature dependence [11], [22], [23] indicate that for typical room temperature variations, i.e., between 15 C to 25 C we should observe gain changes of 1.2% per C for BGO and 0.3% per C for NaI, while LaBr being virtually unaffected. The observed changes, see Fig. 4, would suggest about 15 C increase of crystal temperature, that we find hardly probable, as even a very rudimentary measurement indicated the change of no more than 2 C. Considering the observed long time constants of the gain changes the explanation could be found in the applied electronics. Other parts of the experimental setup, such as amplifiers or HV supplies were not changed in the sequential measurements with all three crystals. In this sense our experiment remains inconclusive regarding the exact reasons of differences in stabilization periods. It points out, however, the importance of careful detector assembling process. The data presented in the Fig. 4 should serve as an example or illustration giving a hint of what kind of an obstacle may be encountered and should be dealt with. C. Efficiency When speaking about efficiency in the context of gamma detection one usually thinks about the probability of registration of gamma rays emitted and gamma rays that hit the detector. Whereas the former aspect is mainly related to the set-up geometry, the latter is conditioned by the intrinsic properties of the applied detector. The absolute efficiency of the detector is important in every application constrained by the measurement time. This parameter gets even more important whenever there are limits to the device physical dimensions, and when the growing price tag prevents from applying just a bigger unit. These are exactly the same requirements that made scintillation detectors so popular in commercial applications, and that made the HPGe detectors visible mainly among the laboratory equipment. In the neutron activation analysis, which itself is a form of spectrometry, the emphasis is also put on the total absorption efficiency, i.e., the probability of registering the full energy of gamma rays that interact with the detector. This quantity is related to the peak-to-total fraction of the particular detector, being in turn a function of the Z value of the mayor element and the size of the detector.



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In general it is next to impossible to obtain clear, monoenergetic spectra because, in the first place, there is a scarcity of natural, monoenergetic sources, and even those existing, such as popular , are not truly monoenergetic. The workaround for this challenge are Monte Carlo simulations of the detectors’ response functions. The modern simulation software, such as Geant 4 or MCNP, is advanced enough to provide high degree of reliability and enough data for working out a complete efficiency analysis of the investigated detector. Unfortunately such simulations are very time consuming and demand very detailed knowledge of the detector geometry. For these reasons we decided to focus on the experimental approach only. Considerably well isolated, the high energy peaks are experimentally available by measuring energy transitions in carbon and oxygen. The 4438 keV gamma line is the result of the decay of C exited state produced in the reaction

in the Pu-Be source. The 6129 keV peak can be easily measured by the activation of oxygen which is abundant in water. The knockout reaction

yields the nitrogen isotope, which 7.2 s half-life enables easy isolation of the interesting transition in oxygen, as long as the neutron generator is suitable for working in the pulsed mode. This gamma line is obtained from the beta decay of nitrogen produced in the reaction

leading to the excited state of oxygen. The spectra of carbon and oxygen, visible in the Fig. 5 were accumulated in the same experimental conditions, and are normalized for time and neutron flux, respectively. We made an attempt to compare two measurable quantities related to the efficiency. The first quantity was the number of counts in the full energy absorption peaks of BGO and LaBr relative to the number of counts in the full energy peaks of NaI(Tl) for both 4.4 and 6.1 MeV energies. The second quantity, referred to as Peak-to-Total ratio, is the ratio of number of counts in this full energy peak to all counts in the selected region. In the perfect case this quantity describes the ability of a detector to absorb all the energy of the interacting gamma ray. In this paper we applied a pragmatic approach to set the lowest limit above 2.7 MeV to avoid the delicate problem of precise background subtraction. The data are presented in Table III and can be immediately compared to the photoelectron numbers and energy resolutions of tested crystals measured at chosen energies and listed in Table IV. Practical impact of the numbers presented in Table III can be seen particularly clearly in the oxygen spectra in the right panels of the Fig. 5. The weak 7115 keV line and its 6607 keV escape peaks (4.8% compared to 66.2% of the main 6129 keV line) are plainly visible in the LaBr detector, and are hardly noticeable in BGO. In the BGO spectrum, small, high energy peaks are smeared over the wider energy region range and are hidden

Fig. 5. The comparison of the detectors response to the 4438 keV line from carbon (left panels) and, mainly, the 6129 keV -line from oxygen (right panels). The intensity of the 7115 keV line in oxygen, which is clearly visible only by the lanthanum bromide detector, is only 7.25% of the stronger line. , The apparent baseline above 7115 keV, visible in the BGO spectrum of is the result of full absorption of cascades de-exciting higher energy levels . The chance of full absorption in BGO that are also fed during -decay of is significantly higher than in other crystals, because the activated oxygen comprises BGO itself. TABLE III A COMPARISON OF EFFICIENCY PERFORMANCE TESTED DETECTORS EFFICIENCIES

OF THE

TABLE IV A COMPARISON OF PHOTOELECTRON NUMBERS, ENERGY RESOLUTION OF TESTED SCINTILLATORS

in a higher background. This example clearly shows that the superior absolute efficiency of BGO cannot compensate other deficits of this scintillator. At this point it is probably worth to

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TABLE V TIME RESOLUTION OF THE TESTED DETECTORS

Fig. 6. Examples of time spectra measured with the investigated detectors. Left panels show time spectra of - coincidences of 1172 keV and 1333 keV gamma rays of Co, while right panels depict n- and -n coincidences in Pu-Be source. The spectra were measured with, starting from top, BGO coupled with XP5300B PMT, LaBr coupled with XP5700B PMT, and NaI(Tl) coupled with XP3312 PMT.

note that unlike unsatisfactory energy resolution, the smaller absorption efficiency can be compensated by longer measurement time, more compact geometry or larger scintillator.

n- and -n coincidences resulting from the detection of a neutron in one of the crystals. Since the source was placed near the BaF crystal, the neutrons’ TOF to the reference detector should not, in principle, affect the left, narrow peak (BaF detector is a small crystal, therefore the neutron TOF spread due to all possible interaction points inside the crystal is smaller). The accompanying, broader, right peak is the result of the opposite situation when neutrons triggered a ’stop’ signal in the tested gamma detector (larger crystal yields a larger TOF spread). In order to separate both peaks, the tested gamma detector was placed up to about 50 cm away from the source. Considering the fact that the velocity of 14 MeV neutrons is about 5 cm/ns one can easily notice the practical limitation of any 3D, the NAA based scanner that has one of the described detectors applied in its design. Nevertheless, the LaBr detector shows much better performance in this category than its competitors. BGO, mostly due to the low light output and long decay time, is in a great disadvantage when compared with the other detectors.

D. Time Resolution Application of the tagged 14 MeV neutron generator, i.e., the one with the dedicated associated alpha-particle detector, offers the opportunity of creating the device capable, at least in principle, to obtain the 3D visualization of the investigated object. However, taking the advantage of any time-of-flight technique, requires a detector of sufficiently high time resolution, as the latter has a direct impact on the spatial resolution. Since we were testing the complete detectors, i.e., scintillators coupled to their photomultipliers with unmodified voltage dividers, the achieved results are significantly worse than those obtained with the dedicated timing PMTs. In order to make our measurements as close as possible to real application scenario we were measuring the n-gamma coincidences in a fast-slow setup using the Pu-Be source. The additional measurements with the Co source were performed to provide the reader with numbers that may be compared with the results of the dedicated timing measurements [4], [24]–[26], see Fig. 6 and Table V. Typical fast-slow arrangement consists of a tested and a reference detectors. A 25 mm in diameter and 15 mm long BaF crystal coupled to a Photonis XP20Y0QDA photomultiplier served as a reference detector. Its time spread of ps, measured for the coincidences in Co and reported in [27], is negligible comparing to the time resolution of the tested detectors. The panels in the right column in the Fig. 6 depict the time spectra of the coincidences measured with a Pu-Be source. These are mainly

III. CONCLUSIONS When starting our measurements we were to some extent biased by the results described in the earlier paper [4]. In fact we expected that BGO will display acceptable performance in most categories while outclassing other scintillators as the most efficient one. The practical requirements, however, and the efficiency in isotope identification in particular, revealed that the high full energy absorption efficiency does not translate well into an ability to separate the energy lines. Software analysis procedures, in general, do not like high uncertainties and deal poorly with low statistics. Even though BGO scintillators have impressive count rates, the counts in the regions of interest are ambiguous and affect the confidence level of the final identification result. The performance of LaBr crystal was seen to be quite close to BGO when only the full absorption peaks were taken into account. If the fact is considered that the process of pair creation becomes dominant in high energy region, the ability to precisely identify the first and the second escape peaks may, and should be, taken into account. In consequence the total amount of useful information one can get during the given period of measurement with the LaBr detector is higher than that obtained with any other of the tested detectors. In other words, any analyzer, human or software, requires less acquisition time to yield the result with the desired accuracy. Other factors, such as temperature insensitivity, radiation hardness and



response time favor lanthanum bromide as well. Among disadvantages are crystal’s highly hygroscopic properties and its cost. The latter, however, is expected to decrease once the patents expire and the world production increases. The original patent [28] was published on 8-th December, 2004 and accordingly to EU law expires after 20 years. Definitely it is a lot of time to let all the designs to mature and in the meantime any instrument with a large lanthanum bromide scintillator is expected to be expensive.

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