Development of Neutron Detector Arrays for Neutron

Development of Neutron Detector Arrays for Neutron-Induced Reaction Measurements

Abstract—The outgoing neutron energy spectra from neutroninduced fission of various actinides are important for basic understanding of the fission process near the scission point as well as playing a large role in neutron transport codes, which are heavily relied upon in the design of advanced nuclear reactors and simulations of critical assemblies. The reliability of the results of neutron transport models is a strong function of the quality of the nuclear data used as input. Currently, the world’s experimental database of fission neutron spectra is severely incomplete (especially for higher incident neutron energies) with large uncertainties in key portions of the outgoing energy spectra. Many transport codes use evaluated data libraries, which are based on the approach of the Los Alamos Model. Other theoretical models have been developed, but the available data can not distinguish the results of different models (as is the case for 239 Pu). Better measurements are needed for all incident and outgoing neutron energies, but most urgently in the low-energy (below 1 MeV) and high-energy (above 6 MeV) portions of the outgoing spectra where theoretical model results differ greatly. We present the design considerations (and some characterization results) of the two Chi-Nu neutron detector arrays: one array of 6 Li-glass detectors and one array of liquid-scintillator detectors. These detector arrays are being constructed to meet the challenge of measuring the prompt fission neutron spectra (for a few common actinides) to a higher accuracy and precision than achieved previously and over a larger incident energy range than has been covered by previous experimenters. We see a significant reduction in neutron-scattering backgrounds with our new array designs.

I. I NTRODUCTION High accuracy and high precision measurements of the prompt fission neutron spectra (PFNS) are required to validate theoretical models and provide quality data for nuclear data applications. A program to carry out these high fidelity measurements for a wide range of incident neutron energies using a spallation source of fast neutrons is underway at the Weapons Neutron Research (WNR) facility, which is situated at the Los Alamos Neutron Science Center (LANSCE) at Los Alamos National Laboratory. The available data on the PFNS are few, as well as, very imprecise and discrepant in certain regions of the energy spectrum. These problems in the current PFNS data are exemplified by considering the neutron-induced fission of 239 Pu, where B. A. Perdue, R. C. Haight, H. Y. Lee, T. N. Taddeucci, J. M. O’Donnell, M. C. White, N. Fotiadis, M. Devlin, J. L. Ullmann, A. Laptev, T. Bredeweg, M. Jandel, R. O. Nelson, and S. A. Wender are with the Los Alamos National Laboratory, Los Alamos, NM 87545 USA. C. Y. Wu, E. Kwan, A. Chyzh, R. Henderson, and J. Gostic are with the Lawrence Livermore National Laboratory, Livermore, CA 94551 USA.

Ratio to Maxwellian (T=1.42)

B. A. Perdue, R. C. Haight, H. Y. Lee, T. N. Taddeucci, J. M. O’Donnell, M. C. White, N. Fotiadis, M. Devlin, J. L. Ullmann, A. Laptev, T. Bredeweg, M. Jandel, R. O. Nelson, S. A. Wender, C. Y. Wu, E. Kwan, A. Chyzh, R. Henderson, and J. Gostic 1.4 1.2 1 0.8 Knitter, 1975 (0.215 MeV) Staples, 1995 (0.5 MeV) Lajtai, 1985 (thermal) Boytsov, 1983 (thermal) ENDF/B-VII.0 Talou Monte Carlo, 2011

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Fig. 1. (Color online) Available data on the prompt fission neutron spectra from the neutron-induced fission of 239 Pu plotted as a ratio to a Maxwellian spectrum with T = 1.42 MeV. The data are compared to predictions from ENDF/B-VII.0 (based on the Los Alamos model) and a Monte Carlo fission model of Talou et al.

very few measurements have been done with fission induced using MeV neutrons. More data are available at lower incident energies (below 0.5 MeV), and Fig. 1 shows the results of a sampling of these measurements [1]–[4] compared to the ENDF/B-VII.0 evaluation [5] and a theoretical Monte Carlo model [6]. All the quantities in the figure are presented as a ratio to a Maxwellian functional shape with T = 1.42 MeV for the sake of clarity. All the data shown agree within uncertainties in the range of 1 – 6 MeV, although the error bars are still relatively large. In contrast, the data above and below this region differ greatly in shape with even larger uncertainties. It is also in these high- and low-energy regions that the theoretical curves diverge in their predictions. (The ENDF/B-VII.0 evaluation is based on the Los Alamos model [7]). In these proceedings we describe the design details, resulting in significant background reduction, and the results of some characterization analyses for the two Chi-Nu detector arrays that will be used to perform high quality measurements of prompt fission neutron spectra. We are constructing an array of 6 Li-glass detectors to detect neutrons below ≈ 1 MeV down to possibly 50 keV and an array of liquid scintillators for detection of neutrons above ≈ 600 keV. The lower limit (50 keV) for the 6 Li-glass detectors reflects the small number of fission neutrons at this energy and below, the need for energy bins of narrow width, and the limiting effects of room-

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10 Fig. 3. (Color online) Layout of the new building at the WNR facility and the location of the Chi-Nu beam line.

Fig. 2. (Color online) MCNPX calculations illustrating the effect of differing floor types on the deduced neutron energy spectrum from 252 Cf. In this graph, the “neutron energy” is calculated from the measured time of flight over a one-meter flight path from source to detector. Scattered neutrons have a longer flight path and therefore appear in the lower energy portion of the spectra. Curve 1 is the spectrum for a bare 252 Cf source. Curve 2 is the spectrum for a solid concrete floor. Curve 3 is the spectrum for a basement below the source and an aluminum bar grating, with dimensions 1” × 1/8” and 1-3/16” spacing, covering the top of the basement. Curve 4 is the spectrum for a basement below the source and no type of floor covering the basement.

return neutron background. II. FACILITY

AND ACTINIDE S AMPLE C ONSIDERATIONS

D ESIGN

When designing and constructing an experiment other important aspects should be considered along with those of the detectors. One of the major challenges with detecting low-energy neutrons is a background of neutrons that have scattered one or more times from the experimental environment, which includes the air and other materials in the room. This background can severely limit the ability to detect neutrons at the lowest energies. This point is illustrated in Fig. 2 where MCNPX [8] neutron energy spectra calculated from the time-of-flight of neutrons emitted by a 252 Cf are compared for different types of flooring placed approximately one meter underneath a point detector. The neutrons are tallied in a hypothetical detector located in the horizontal plane one meter from the source. The take-away point of this figure is that a solid concrete floor (the curve labeled “no pit”) scatters neutrons into the detector creating a large background below 200 keV. Thus our source and detectors need to be located farther than one meter from the concrete floor of the experimental area. Since the height of the WNR beam lines is fixed at approximately one meter, a new beam line has been constructed with a basement of dimensions 5.48 m × 5.48 m × 2.13 m deep built below the Chi-Nu location in a new building (see Fig. 3). An aluminum bar grating will be placed over the basement to support the detector and actinide sample stands. The MCNPX results for a bar grating over the basement are shown in Fig. 2. These results indicate that using the basement and bar grating

will result in much lower background levels, which could allow us to reach neutron energies as low as 50 keV. The actinide samples that are used in our experiments are enclosed in parallel-plate avalanche counters (PPACs). The construction of these PPACs, designed and built entirely at Lawrence Livermore National Laboratory, addresses the issues of beam-related neutron background and of neutron multiple scattering by using as little material as possible [9]. Fig. 4 shows the 235 U PPAC along with a schematic of the structure of the foil stacks inside. The PPAC detects a fission event by collecting ionization charges (and their secondaries) created in the region between the cathode and anode. The PPAC shown encloses ≈ 100 mg of sample material deposited as 4 cm diameter circles on each side of ten titanium foils [10]. Each 3 µm thick titanium foil has 400 µg/cm2 of material electroplated on each side. Each aluminized mylar foil is 1.4 µm thick. The platinum foils, used to prevent cross talk between adjacent PPAC cells, are 5 µm thick. Thus the amount of non-actinide material in the beam is reduced by about two orders of magnitude relative to older fission chambers that were used in the past at WNR [11]. The PPAC support stand is made entirely of aluminum and has been engineered and machined such that the minimum amount of material is used while still maintaining its structural integrity. The adjustment bolts are also made of aluminum rather than steel. III. L OW-E NERGY D ETECTORS Although liquid scintillators have a reasonable efficiency (≈ 15–30 %) due to the large n-p scattering cross section, discriminating neutrons from γ rays using the shapes of detector pulses becomes difficult at low energies. So to measure the PFNS below 1 MeV, we are constructing an array of 6 Li-glass scintillator detectors. These scintillators produce light via the 6 Li(n,α)t reaction, which has a positive Q value of 4.8 MeV. Much development work has been completed and is described by Lee et al. [12] [13]. Detectors of 6 Li glass have been tested using a 252 Cf spontaneous fission source and a 235 U PPAC in the WNR neutron beam. Analysis procedures and MCNP [14] Monte Carlo

Fig. 5. (Color online) Energy spectrum from a 6 Li-glass detector viewing the field of neutrons from a 252 Cf source compared to an MCNP model result.

Pt foil Al-mylar foil Al-mylar foil Cathodes Sample Ti foil Al-mylar foil

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Fig. 4. (Color online) A 235 U parallel-plate avalanche counter (PPAC). On the top, the PPAC is shown placed in the neutron beam incident from the left. In the middle, the PPAC is shown with its cover removed. On the bottom, a schematic of a single foil stack is shown; the PPAC has ten foil stacks.

models have been developed to select prompt neutrons from fission, estimate backgrounds, and apply geometry corrections. Fig. 5 shows the energy spectrum calculated from time-offlight acquired with a Chi-Nu 6 Li-glass detector viewing the field of neutrons produced by a 252 Cf source compared to the results of a detailed MCNP model. Each detector is 10 cm × 18 mm (diam. × thick.) and optically coupled to a 12.5 cm diameter Hamamatsu R1250A photomultiplier tube (PMT). It was determined that the detector mounts required redesign from analyses of the experimental and MCNP Monte

Fig. 6. (Color online) A 10 cm × 18 mm 6 Li-glass scintillator mounted on a 12.5 cm diameter Hamamatsu R1250A PMT. The detector is shown mounted in our new lower-mass holder and stand.

Carlo data; the previous detector mounts [11], which were mostly comprised of hydrocarbon material, contributed to backgrounds at the lowest energies due to neutron multiple scattering. This background is significant and can be identified in Fig. 5 as the sharp rise below 0.1 MeV. The 6 Li-glass detector assembled with the updated detector mounts is shown in Fig. 6. The redesigned detector mounts use much less material, of which the majority is aluminum. The neutron multiple-scattering backgrounds are reduced by approximately a factor of 2 at 50 keV using the new mounting scheme, which has no hydrocarbon material (see Fig. 7). In the tests with the 235 U PPAC in the neutron beam and a single 6 Li-glass detector, we have measured the prompt fission neutron spectrum down to approximately 60 keV outgoing neutron energy. (An identical 7 Li-glass detector was used to measure the γ-ray background.) This spectrum was averaged over all incident neutron energies due to limited statistics. In the final configuration we will field an array of 20 6 Liglass detectors (and one or more 7 Li-glass detectors), placing them at a flight path distance of 40 cm to increase solid angle

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En (MeV) Fig. 7. (Color online) Comparison of the 252 Cf energy spectrum measured with a 6 Li-glass detector mounted with the former hydrocarbon-based mounts and the new aluminum-only mounts. Scattered neutron background is reduced by approximately a factor of 2 at 50 keV using the aluminum mounts. The residual tail is due to scattering from the walls, floor, etc. Fig. 9. (Color online) Conceptual drawing of the liquid-scintillator detector stands. See the text for a detailed description.

Fig. 8. (Color online) Prototype of a 17.8 cm × 5 cm EJ309 liquid scintillator mounted on a 12.5 cm diameter Hamamatsu R4144 PMT.

coverage. IV. H IGH -E NERGY D ETECTORS To measure the PFNS above 600 keV up to approximately 12 MeV, we are constructing an array of liquid-scintillator detectors. These scintillators produce light via neutron scattering from protons and carbon nuclei, with the n-p scattering being the dominant mechanism producing scintillation light in this neutron energy range. Previously we have used EJ301 liquid scintillators with diameters of 12.5 cm and depths of 5 cm [11]. To increase our efficiency for detecting neutrons we have chosen new larger diameter (17.8 cm) detectors filled with EJ309 liquid available from Eljen Technology [15]. We chose this liquid because of its higher hydrogen content and its larger hydrogen-to-carbon ratio. A prototype of the detectors we will field is shown in Fig. 8. We are currently constructing new detector stands. The detectors will be mounted on rigid arcs with a radius that will place the face of the detectors at a 1 meter flight path. In the final design we will field 54 detectors 17.8 cm × 5 cm (diam. × thick.) mounted on the strong and relatively low

mass structure. Fig. 9 shows the structure, where there are two identical stands facing one another. Each stand has three arcs, and 9 detectors can be mounted on each arc. The detector positions are spaced 15◦ apart with the first position at 30◦ and the last position at 150◦ . The arc spacing in the azimuthal angle is adjustable (for the non-horizontal arcs) from a minimum of 33◦ to a maximum of 45◦ . All of the features of the detector stands play a significant role in our goal of performing measurements of the PFNS to a higher precision than has been achieved before. The stands use a minimum of material while maintaining strength and holding the detectors firmly in place. They also have been engineered to hold as many detectors as possible while positioning the detectors as far away from the floor of the experimental area (i.e., on the top half of each stand). This is crucial to minimize neutron return from the floor. This feature combined with the basement (discussed in Sec. II) built into the floor underneath the experimental setup gives significant gains in reducing neutron multiple-scattering backgrounds. This arrangement of detectors allows us to implement the geometries in Monte Carlo codes such as Geant4 [16] [17] and MCNP. The use of Monte Carlo codes is very important in characterizing the detectors response to various fields of neutrons and γ rays. We require that the efficiencies of the detectors be known very well, and we aim at the 2% value reported by Weber et al. [18]. This requirement forces us to have more precise knowledge of many detector characteristics such as light output response curves for protons and carbon nuclei, pulse height spectra for mono-energetic neutrons, efficiency curves as functions of threshold, and potential sources of backgrounds. Each of the above categories must be explored in detail to achieve our high precision goal. While we are primarily reporting on our background-reduction efforts through the design of our experimental area, actinide sample containers,

TABLE I N EUTRON IN - SCATTERING EFFECTS FOR EACH COMPONENT.

Fig. 10. (Color online) A representation of the sources of detector cross talk. Each blue point indicates the initial direction (θn , φn ) of a neutron that was scattered into the detector at θ = 135◦ , φ = 33◦ . Although there is a contribution from all detectors and the air in the room, the majority of the in-scattering is due to the nearest neighbors.

and neutron-detector arrays in these proceedings, we have begun to address the subtleties in characterizing our detectors at this precise level. We have in house a 252 Cf ionization chamber manufactured by Oak Ridge National Laboratory, and we will use this chamber to perform more measurements on the detector response in pulse height and time of flight to this source of neutrons. In addition to 252 Cf ionization chamber measurements, we will perform measurements with monoenergetic neutrons produced either with associated-particle techniques [18] [19] or at other facilities where mono-energetic beams are available. We believe that high quality measurements using a 252 Cf ionization chamber and mono-energetic neutrons coupled with detailed Monte Carlo simulations will allow us to further characterize our detectors to a higher degree. As we have discussed in this section, great care has been taken to minimize the amount of material near the detector in an effort to reduce the neutron multiple-scattering backgrounds. To increase our count rates, we are employing many detectors placed as closely together as space allows. By doing this we introduce hydrogenous material close to each detector, namely its nearest neighbors. Neutrons incident on one detector can scatter from its active volume, deposit some energy, and then strike another detector in the array where it can also deposit energy. This is called detector cross talk, and we are using a Geant4 simulation to investigate the backgrounds introduced by it. Fig. 10 illustrates the neutron multiple scattering from all detectors into a single detector using a Geant4 simulation. In this simulation, the detectors were placed in a room filled only with air and no other volumes. A second simulation was executed with the detectors stands included in the room to determine their contribution to the scattering backgrounds. The initial neutrons were generated at a point at the center of the array of 54 detectors, the origin, and in a random direction (θn , φn ). In

Component

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the figure, the initial φn is plotted versus the initial θn and the detector is located at θ = 135◦ and φ = 33◦ . The solid angle of each detector is represented by the red ellipses. Each green point in the scatter plot represents a neutron that was initially directed toward the detector and was detected (i.e., a nonscattered neutron). Each blue point represents a neutron that was initially directed toward another detector but was detected by the detector at θ = 135◦ and φ = 33◦ (perhaps in addition to being detected by the detector in its initial direction). This scatter plot shows that the majority of the scattered neutrons come from the detectors that are nearest. In this simulation run, about 5% of the total number of neutrons detected are scattered into the detector from all of the others and the air in the room. Table I gives a breakdown of the neutron inscattering effects from the components of our detector array. The detector-to-detector crosstalk produces the largest inscattering contribution at 4.3 %, while the stands contribute at the 1.0 % level (to which the scattering from the air in the room is comparable). The crosstalk and stand in-scattering contributions overlap each other in the time-of-flight spectrum with the crosstalk distribution containing approximately five times as many counts as the distribution of scattering by the stand. From these simulation analyses, we conclude that the array stands contribute a small amount (1 %) to the neutron multiple-scattering backgrounds and there are larger sources of scattering backgrounds such as the detector crosstalk that should be studied in greater detail. V. C ONCLUSIONS High quality measurements, with great improvements in both accuracy and precision, are needed for the prompt fission neutron spectra over a wide range of incident neutron energies. To impact evaluated data libraries such as ENDF, the shape of the spectra should be measured to 5 % in key portions of the outgoing neutron energy range. Reaching this goal is challenging, and the challenge is in measuring a reaction with low event rates at low outgoing neutron energies while maintaining minimal neutron-scattering backgrounds at a facility that produces neutron beams with a wide range of energies. This presents many technical issues to be resolved. Our previous work [11] was limited by large neutron multiplescattering backgrounds from the former in-beam fission chamber, detector mounts, and experimental area (including its walls, floors, and miscellaneous items). In response to these limitations, the entire experiment was redesigned resulting in a significant reduction in neutron multiple-scattering backgrounds. A new building with a new

Chi-Nu experimental area is being constructed. The experimental area includes a 2.13 m deep basement that reduces neutrons scattered from the floor by approximately a factor of two at 50 keV. New actinide fission chambers (PPACs), designed by Lawrence Livermore National Laboratory, not only reduce the amount of extraneous material in the beam by about two orders of magnitude but also significantly improve the timing. The 6 Li-glass detector mounts were redesigned to eliminate hydrocarbon material near the scintillators, resulting in a reduction in the low-energy background tail of the outgoing neutron energy spectrum by approximately a factor of two at 50 keV. Our new liquid-scintillator array stands can support up to 54 detectors and contribute only a 1 % inscattering contribution to any one detector. This contribution from the stand is comparable to the contribution of the air in the room. Our experimental redesign has lowered the number of potential sources of background, and we are confident that this work will ultimately result in a better understanding of the nuclear fission process and in a significant contribution to the available data used in evaluated data libraries such as ENDF. In the current run cycle we will employ our new 6 Li-glass detector array on the new beam line over the basement in the newly constructed building. The array of liquid scintillators is scheduled to be used on the beam line in the following run cycle.

[6]

[7]

[8] [9]

[10]

[11]

[12]

[13]

ACKNOWLEDGMENTS This work benefited from the use of the LANSCE accelerator facility and was performed under the auspices of the US Department of Energy by Los Alamos National Security, LLC under contract DE-AC52-06NA25396 and by Lawrence Livermore National Security, LLC under contract DE-AC5207NA27344.

[14]

[15] [16]

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