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A new fission chamber dedicated to Prompt Fission Neutron Spectra measurements
Nuclear Instruments and Methods in Physics Research A 833 (2016) 1–7
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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A new fission chamber dedicated to Prompt Fission Neutron Spectra measurements J. Taieb, B. Laurent n, G. Bélier, A. Sardet, C. Varignon CEA, DAM, DIF, F-91297 Arpajon, France
art ic l e i nf o
a b s t r a c t
Article history: Received 8 February 2016 Received in revised form 1 June 2016 Accepted 30 June 2016 Available online 1 July 2016
New fission chambers dedicated to Prompt Fission Neutron Spectra measurements with the time-offlight technique have been developed. The actinide mass embedded in the chamber was maximized, while the alpha-fission discrimination and the time resolution were optimized. Moreover, to reduce the neutron background and spectra distortions, neutron scattering with the materials were minimized by the choice of material and structure. These chambers were then tested and validated during tests and inbeam experiments. & 2016 Elsevier B.V. All rights reserved.
Keywords: Fission chamber Alpha-fission discrimination Prompt Fission Neutron Spectra
1. Introduction Prompt Fission Neutron Spectra (PFNS), and neutron data measurements in general, are in the center of interest for nuclear reactors, global security or fundamental understanding of the fission process. As far as the PFNS are concerned, evaluated data show discrepancies on the low (below 1 MeV) and high (above 5 MeV) energy parts for different major and minor actinides. In addition, this observable is important for the understanding and modeling of the fission process. Unfortunately the experimental data are rather scarce, specifically for the high energy neutron induced fission reactions. Recent publications [1,2] point out the needs on new experimental data to produce new PFNS evaluations with uncertainties for actinide nuclei in accordance with the Coordinated Research Project (CRP) “Evaluation of Prompt Fission Neutron Spectra of Actinides” established by the IAEA Nuclear Data Section in 2009 [3]. With this mind, we decided to launch a new measurement campaign based on improved experimental tools. In our experimental approach, PFNS are measured with the time-of-flight technique between a fission chamber and neutron detectors, set at a flight distance close to one meter. The energy resolution is then related to the time resolution of the setup, and the accuracy of the data depends on the level of background in fission and neutron detectors. Our available fission chambers were built in 70's and then degraded with time. Moreover, they were not optimized for this type of measurements. In addition, we took advantage of the n
Corresponding author. E-mail address: [email protected] (B. Laurent).
http://dx.doi.org/10.1016/j.nima.2016.06.137 0168-9002/& 2016 Elsevier B.V. All rights reserved.
improvements in electronics to develop new tools. New fission chambers, dedicated to this type of studies, were then developed, to complete the development and characterization of neutron detectors to reduce the detection threshold in PFNS measurement [4]. The development of these fission chambers was focused on different points: unambiguous alpha-fission discrimination to ensure an accurate fission trigger; excellent time resolution to perform high resolution time-of-flight measurements; very low quantities of structure material to avoid neutron spectra distortion; and a reasonable amount of actinides to obtain a sufficient count rate for high statistics measurements. A version of the chamber, containing 252Cf and dedicated to calibration measurement was also built with the same characteristics. All these requirements will be detailed in the following sections.
2. Structure of the chamber The detector is a multi-layer fission chamber containing a stack of axial ionization cells. The cathodes are referenced to the ground, while the anodes are polarized to a potential which depends on the gas mixture, its pressure and the gap between plates. In order to reduce the amount of material, and therefore limit the neutron scattering, as thin as possible hydrogen-free structure materials are used for all the parts of the chamber. Figs. 1 and 2 show a picture and a sketch of the chamber. The housing is made of 1.5 mm thick aluminum tube of square section. The external dimensions are 80 × 80 × 220 mm3. The housing is closed by two aluminum flanges, supporting a 100 μm thick, 38.5 mm diameter titanium windows in order to minimize the material budget in the incoming neutrons path.
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of structural material, the possible distortion on Prompt Fission Neutron Spectra of 238U due to the chamber can be corrected using the 252Cf reference.
4. Gas The choice of the gas is of utmost importance to fulfill the detection specifications. It has to exhibit a high drift velocity and provide a good alpha-fission separation. For those reasons, we chose to use Tetrafluoromethane with the newly developed fission chambers.
Fig. 2. Sketch of the fission chamber with the main characteristics.
In the chamber, the actinide backings consist of 50 μm thick, 59 mm diameter titanium disks, held by four insulating rods made of Torlons [5] exhibiting a hydrogen content of 2%. The cathodes are connected to a common ground while the anodes are paired two by two to SMA feed-through connectors. Dedicated preamplifiers (see details in Section 5) are directly connected to the other side of these connectors.
3. Actinide deposits The studied actinides emit alpha particles and undergo fission either spontaneously or induced by a neutron absorption. Both events, alpha emission or fission, produce signals related to the energy loss in the gas of the chamber. It is crucial to discriminate between those two classes of events to trigger the acquisition system only when a fission occurs. In order to reach a good alphafission discrimination, the thickness of the deposit should be low. However this leads to a severe limitation for the total possible embedded actinide mass. In order to satisfy these two constraints, several couples of anode/cathode were stacked in the chamber, while keeping a reasonable number of electronic channels. Two fission chambers were constructed and tested. The first prototype was equipped with one cathode supporting a 25 mm diameter electro-deposit of 252Cf sample. Since 252Cf is both an alpha emitter and undergoes fission spontaneously, it is a good candidate to test the response of the fission chamber to those competing decay modes. Fission neutron detection efficiency measurements of neutron detectors were also made with this chamber, since PFNS of 252Cf is well known and recommended as a evaluated standard [6]. The second fission chamber was built to contain 238U and used on neutron beam facilities. A total of 72 deposits (33 mm diameter, 5 mg) were produced by electroplating at the CACAO [7] facility for a total of 360 mg of 238U. The uranium was deposited on both side on each anode and cathode, except on both extremities, where the deposits were only on the internal sides. Since both fission chamber (252Cf and 238U) are similar in terms
4.1. Drift velocity The time resolution is a key point in time-of-flight measurements. Since the fission chamber acts as the “start” of the timing measurement, a good intrinsic resolution is needed. When an alpha emission or a fission occurs, emitted particles ionize the gas and electron-ion pairs are created. The electrical signal on the electrodes is mainly due to the drift of electrons towards the anode. The faster the electron, the shorter the rise time of the signal will be. This is obtained choosing an ionizing gas showing a high electron drift velocity, at reasonable electric field, such as Tetrafluoromethane (CF4). Fig. 3 compares the electron drift velocity, obtained from Garfield [8] simulations, as a function of electric field for different gases usually used in fission chambers like P10 (90% Ar, 10% CH4) or P20 (80% Ar, 20% CH4), and CF4. Although CF4 requires a higher electric field to reach high drift velocity, the value obtained at 1.3 kV/cm, 11 cm/μs, is roughly a factor of two higher than that typically reached by other detectors. The CF4 gas exhibits also the advantage to show a higher density compared to most gases used in ionization chambers. Hence it allows a small gap distance between anodes and cathodes to be kept even at atmospheric pressure. Thus, we could obtain a compact, and light weight fission chamber despite a large number of plates (37). However, this gas has the disadvantage to be very sensitive to the oxygen pollution, requiring a permanent gas flux in the chamber. A regulation gas system is then used, to insure a constant flux and a pressure 50 mb above the atmospheric pressure during the measurements. Once the gas was chosen, the other parameters, such as gap between electrodes, voltage value… should be fixed accordingly to the gas specificities as described in the following section.
Drift velocity (cm/µs)
Fig. 1. Pictures of the fission chamber (outside: left and inside: right). The electronics cards are the motherboard hosting the preamplifiers (not in place). On the right side, cathodes are grounded while anodes are connected to the feed-through connectors.
12
10
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6
CF4
4
P10
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P20
0
0
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600
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E field (V/cm) Fig. 3. GARFIELD simulation [9] of the electron drift velocity (cm/μs) as a function of electric field (V/cm) at atmospheric pressure for P10, P20 and CF4. The dot and dash lines represent the value of field (and corresponding drift velocity) chosen for the operation of the fission chamber.
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4.2. Stopping power
5. Electronics and acquisition system
In order to separate properly alpha decays from fission events, the electric charge induced by an alpha decay in the gas should be lower than that of fragments emitted in the fission process, for all emission angles. The induced charge qind on the anode is depending on the gap D between the anode and the cathode, the charges Q0 created by the particles and the charges path length L in the gas perpendicular to the electrodes, following the RamoShockley theorem:
5.1. Preamplifier
qind = Q 0 ×
L D
(1)
The most unfavorable case occurs when alpha particles are emitted parallel to the deposit (the induced charge correspond then to the full alpha energy ( E6 MeV which is around the maximum alpha energy for actinides)) while fragments are emitted perpendicular. In that case, assuming a constant stopping power along the trajectory (the charges are created in average in the middle of the gap: L = D/2), the fragment induced charge is then equal to Q 0/2. Thus, the fission fragment should lose at least 12 MeV, to induce a charge equivalent to the worst case alpha emission. Moreover, in case of high alpha activity, alpha pileup leads to a degradation of the separation. To limit the overlap between alphas signal and fission signal, we designed the chamber so that fission fragments show an energy loss of at least twice the maximum induced charge of an alpha. For a light fission fragment (Z¼ 36) at 120 MeV, LISE þ þ [10], with the method from [11] for energy loss calculations show that an energy loss of 24 MeV in CF4 at atmospheric pressure correspond to a path of 1.5 mm. To be conservative, and due to mechanical considerations, the gap was then chosen equal to 2.5 mm. This value is a good compromise between the discrimination performances, the stackup capacitance and the total length of the chamber. Considering Fig. 3, to reach a drift velocity of 11 cm/μs, an electric field of 1300 V/cm has to be applied between anodes and cathodes. With the chosen gap at 2.5 mm, a potential of 325 V on the anodes is applied, the cathodes being grounded. A drift velocity of 11 cm/μs means also a total electron drift duration of 23 ns, leading to a good intrinsic time resolution of the fission chamber, providing that the electronic chain does not add an extra contribution.
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Good drift velocity in the gas is not enough to ensure a good time resolution, the electronics should be adapted to fast signals. A dedicated preamplifier was developed to meet the requirements of this fission chamber. As can be seen on Fig. 4, a specific motherboard hosting three preamplifiers has been designed. Two low voltages and one high voltage are common to the three preamplifiers. They are provided through LEMO connectors. The charge signal outputs are connected through SMA connectors while the anode signals from the fission chamber are collected on the back side of the motherboard, directly via the feed-through SMA connectors of the fission chamber. The absence of cables between the chamber and preamplifiers reduces the input capacitance and improves the signal over noise ratio. The preamplifier intrinsic rise time has to be short to preserve the good timing response of the chamber, and the duration of the signal has to be short enough to avoid excessive pileup. The choice was to develop charge preamplifiers (to ensure good amplitude discrimination) with a short RC decay time constant to avoid the saturation of the preamplifier in case of very high alpha activity. Fig. 5 shows a collection of waveforms obtained with the 252Cf chamber. The separation, in terms of amplitude, between alpha and fission signals is clearly visible. The full width at half maximum of the signal does not exceed 700 ns, and the rise time (10– 90%) is of 18 ns. For the 238U chamber, we chose to interconnect two anodes on the same preamplifier to increase the total actinide mass while keeping a reasonable number of electronic channels. 5.2. Digital acquisition system: FASTER The signals from the fission chamber preamplifier are directly sent to the digital acquisition system FASTER (Fast Acquisition SysTem for nuclEar Research), currently being developed at LPC Caen [12]. Signals are digitized by a 500 MHz, 12 bits Analog-toDigital low noise (1.1 lsb rms) Converter (ADC) and processed by real time numerical modules, implemented on FPGAs (Field Programmable Gate Array). The analog bandwidth of the sampling ADCs is limited at 100 MHz thanks to a passive input low pass filter, in order to ensure a good zero time crossing determination on the Constant Fraction Discriminator (CFD) signal (using a 2nd order polynomial interpolation), and to optimize the signal over noise ratio. Finally FASTER can provide a 7.8 ps-accuracy time data. The real time treatment is fast enough to authorize high count rate
Fig. 4. Left: picture of a motherboard hosting three preamplifiers and the different connections (signal input/output, low and high voltages). Right: back side of the same motherboard with the SMA input connectors to the fission chamber.
J. Taieb et al. / Nuclear Instruments and Methods in Physics Research A 833 (2016) 1–7
Amplitude (V)
Amplitude (V)
4
0.12 0.1 0.08 0.06 0.04 0.02
0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 -0.05 -0.01
0
18 ns 0
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0.02 Time (μs)
-0.02 -0.04 -0.06 -0.08 0
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Fig. 5. Preamplifier signal from the fission chamber. Alpha (lower amplitude) and fission events are clearly visible. The inset shows the rise time of about 18 ns, compatible with the drift velocity of the gas for a 2.5 mm gap.
(up to 700,000 events per second). Only relevant parameters (CFD timing, integrated charges, pulse height etc.) are stored. A graphical user interface and an online visualization program on the acquisition computer allow to setup and monitor all experiment parameters. Moreover, by absolute time stamping of each event, the reconstruction of coincidences off-line or a complex analysis is possible.
Scattered/direct neutron
Time (μs) 0.2
0.15
0.1
0.04
6. Simulations of neutron background and prompt neutron spectra distortion When measuring prompt fission spectra, several sources of background and distortion have to be considered. The fission chamber was conceived to minimize the scattering of the beam neutrons on the apparatus and subsequently detected in the neutron detectors by random coincidences. As for fission neutron, their scattering by the chamber structure elements and body was also investigated. In order to estimate neutron scattering on the fission chamber material, Monte Carlo simulations were performed using the MCNPX [13] code. Two components were studied; the first one consisted in placing the chamber into a mono-energetic neutron beam and measuring the fraction of neutron scattered on the structure and detected around the chamber. The second one consisted in simulating a neutron source with an energy distribution which followed a Watt spectrum on one of the deposit in the center of the chamber. We estimate the distortion of this spectrum out of the chamber for different emerging angles. 6.1. Neutron beam scattering The fission chamber is developed to be used in different facilities with collimated/focussed (LICORNE at IPNO, WNR at Los Alamos, NFS at GANIL…) or non-collimated (4 MV Van de Graaff at Bruyères-le-Chatel) neutron sources. When collimated neutron beams are used, only the entrance and exit windows, together with the backing of the deposits, can contribute to neutron scattering. In the other case, the whole fission chamber scatter the neutrons. In order to simulate both configurations, a mono-energetic (from 1 to 15 MeV by 1 MeV steps) uniform cylindrical source of two different diameters is defined. The sources are represented by the cylinders in the scheme of Fig. 6 (upper part). They are characterized by a diameter of 33 mm to match the expected neutron
0.03
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En incident (MeV) Fig. 6. Ratio of scattered neutrons over beam neutrons for a non collimated beam (upper spectrum) and a collimated beam (lower spectrum). The picture on the top part represents the scheme of the simulated configurations, with the two beam diameters and the detection surface (truncated sphere) used for scattering estimations.
beam and a diameter of 140 mm to cover the whole fission chamber. Ratios between neutron passing through the truncated sphere (in the interval 45–135°relative to the beam) and inside one of the cylinder, yielded the scattered neutrons over direct neutrons. The ratio is plotted on the lower part of Fig. 6 for different incident energies. Assuming a neutron beam flux of 106 n/cm2/s, at 2 MeV incident neutron energy (the worst case scenario in term of scattering), the fission rate is of 450 Hz for 360 mg of 238U. Since the average multiplicity of fission neutrons is 2.7 [14], the number of emitted neutron is 1215 n s 1. When the diameter of the neutron beam is 33 mm, for the same neutron flux of 106 n/cm2/s and a scattered ratio of 0.033, the number of scattered neutrons is about 2.8 × 105 per second. However, considering a coincidence windows of 200 ns after each fission event to detect the prompt fission neutron, the number of scattered neutron in the same time is then equal to 25 n s 1, compared to the 1215 emitted prompt neutrons. In the case of non-collimated neutron source, the same
J. Taieb et al. / Nuclear Instruments and Methods in Physics Research A 833 (2016) 1–7 ×10 252
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Fig. 8. Zoom in the ordinate axis of the pulse height spectrum of the 252Cf, showing the very good alpha-fission discrimination. The inset is the ordinate axis log scale of the same spectrum.
90°
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En incident (MeV) Fig. 7. Ratio of the spectra simulated outside the chamber with the Watt neutron source spectrum for the three different angles. The dashed lines represent 7 5% of distortion. The lower threshold is set at 400 keV.
calculation give a number of 2610 s 1 scattered neutrons, compared to the 1215 s 1 prompt neutrons. That means that even with a dedicated fission chamber, a collimated, or focussed, neutron beam is highly advantageous to limit the neutron background in the neutron detectors, whereas extracting properly the prompt neutrons from background neutrons in non-collimated beam configuration is still challenging. 6.2. Prompt fission neutron spectrum distortion For the second type of simulations, the neutron source consists in a typical Watt spectrum placed at the center of the chamber. The energy of the neutrons detected outside the fission chamber was reconstructed from their simulated time of flight at three different angles relative to the beam direction: 0°, 45°and 90°. The ratio of these spectra to the source spectrum, is plotted on Fig. 7. The results show that the spectrum distortion is maximum at 0°. It denotes that the titanium backing (50 μm thick × 36 plates) of the deposit is preponderant in the distortion effects. Therefore, for further improvements, thinner backing (25 μm) will be considered. The prompt fission neutrons emitted in the fission chamber and flying to the neutron detectors placed between 45° and 135°, including 90°, are little affected: less than 5% of distortion between 600 keV and 2 MeV, less than 10% in the whole energy range above the threshold set at 400 keV.
spontaneously. Thus the discrimination between alpha decay and fission events could be studied without the need of a neutron beam. Fig. 8 shows the pulse height spectrum obtained with the FASTER system and illustrates the achievable discrimination. The overlap between the two components is almost null. In order to check the time resolution, a coincidence measurement between the fission chamber and a LaBr3:Ce fast gamma detector was performed. The intrinsic time resolution of the LaBr3:Ce detector was previously measured to be of 370 ps [4,15]. Fig. 9 shows the time-of-flight spectrum obtained between the start signal given by the fission chamber and the stop signal from the gamma detector. The width of the prompt gamma peak obtained represents the quadratic sum of both the LaBr3:Ce and the fission chamber resolutions. From that, a value of 732715 ps (FWHM) for the chamber coincidence peak resolution is extracted, showing its very good timing. 7.2.
238
U fission chamber
Prompt Fission Neutron Spectra measurements were performed on the LICORNE [16,17] beam line at the IPN Orsay laboratory and on the LANSCE/WNR [18,19] facility at Los Alamos, using the 238U fission chamber, in coincidence with liquid scintillator neutrons detectors array. Both anodes and cathodes were coated with a 0.6 mg/cm2 thick depleted uranium, so that the total
Counts
Ratio output/source
1
FWHMFC = 732 ps
3000 2500 2000 1500 1000 500
7. Performances measurements 7.1.
252
Cf prototype
A first fission chamber prototype containing 252Cf as a fissile material was developed and built. The alpha activity of the sample was of 65 kBq. 252Cf is both an alpha emitter and undergoes fission
0
20
20.5
21
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22
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γ time of flight (ns) Fig. 9. Prompt gamma peak time-of-flight spectrum between the 252Cf fission chamber signal and a LaBr3:Ce gamma detector. The fit of the peak (dotted red Gaussian) give a total FWHM of 820 ps, leading to a time resolution of the fission chamber equal to 7327 15 ps (FWHM). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
J. Taieb et al. / Nuclear Instruments and Methods in Physics Research A 833 (2016) 1–7
Counts
6
Table 1 Comparison of the main characteristics of the old and new fission chambers.
without /with incident neutron beam 238
U
1
Gas Structure material Total mass of structure material Housing thickness Beam axis thickness (windows þ backing) Neutron beam scattering Maximum spectrum distortion Timing resolution Alpha-fission discrimination
10
10
fission events
α 0
5
10
15
20
25
×10 30
Charge (a.u.)
Counts
Fig. 10. Pulse height spectra of the 238U, with a low fission event overlap. Contrary to the 252Cf, for 238U the alpha only contribution is easily obtained by running the chamber without incident neutron beam (red dashed spectrum). Counts are normalized to the run duration in order to compare easily both spectra. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
anodes and cathodes coated
10
only cathodes coated 10
10
10
5
10
15
20
25
× 10 30
Charge (a.u.) Fig. 11. Comparison of the alpha-fission discrimination: deposits on anodes and cathodes (full line) and deposits on cathodes only (dashed line).
amount of fissioning material was 360 mg. Two anodes are connected to the same preamplifier, meaning that 8 deposits are summed on one acquisition channel. Due to the low alpha emission, the pileup and count rate remain acceptable. The chamber performances obtained, in terms of alpha-fission discrimination, are very similar to that of the 252Cf one, as shown in Fig. 10. The overlap between alpha and fission events is higher than that of the 252Cf, due to the uranium deposit thickness, but it is still negligible. During the development of the chamber, tests with deposits on cathodes only were performed. The discrimination is even better in this case, as shown in Fig. 11. This configuration could be used in measurements where the number of fissions has to be known very precisely, to the detriment of the amount of actinide, and therefore of the fission count rate. In the case of Prompt Fission Neutron Spectra measurements, the statistics recorded is crucial and the configuration with deposits on both anodes and cathodes were used, using a slightly higher threshold in order to eliminate alpha and non separated fission events.
8. Conclusions New fission chambers were developed to optimize the Prompt Fission Neutron Spectra measurements, to lower the distortion of detected neutron spectra and to improve the time resolution and
Old FC
New FC
P20 (6 b) Stainless steel 4 kg 5 mm 12 mm 30% 20% 5 ns 20%
CF4 (1 b) Aluminum 1 kg 1.5 mm 2 mm 3% 5% 1 ns <2%
alpha-fission discrimination. Not only the fission chamber itself but also the whole experimental system was optimized to achieve very good performances. The Table 1 shows the improvement of the main characteristics of this new fission chamber, compared to our typical old fission chamber. These obtained characteristics allow to perform new Prompt Fission Neutron Spectra measurements with better statistics and better energy resolution. This work showed that with thinner deposits on cathodes only, it is possible to separate completely alpha and fission (see 252Cf tests in Section 7.1). Other experiments, where the number of fission has to be known exactly can be investigate with such a chamber: (n, xn) measurements with a veto fission or fission cross section measurements. However, the work on the fission detection efficiency has to be completed to extract accurate data. The good characteristics of the fission chamber can be also useful for Prompt fission γ-rays spectra measurements, which are very sensitive to distortion on setup material. Specific tests with high activity samples (up to 14 MBq alpha activity) have been performed to estimate the alpha fission discrimination reached in such configuration, and improvements have been made to increase this efficiency. Results will be presented in a forthcoming publication. A fission chamber containing a total amount of 50 mg of 239Pu is planed to be assembled. We intend to use that chamber to perform Prompt Fission Neutron Spectra measurement with high statistics, good time resolution and alpha-fission discrimination capability.
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Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A new fission chamber dedicated to Prompt Fission Neutron Spectra measurements J. Taieb, B. Laurent n, G. Bélier, A. Sardet, C. Varignon CEA, DAM, DIF, F-91297 Arpajon, France
art ic l e i nf o
a b s t r a c t
Article history: Received 8 February 2016 Received in revised form 1 June 2016 Accepted 30 June 2016 Available online 1 July 2016
New fission chambers dedicated to Prompt Fission Neutron Spectra measurements with the time-offlight technique have been developed. The actinide mass embedded in the chamber was maximized, while the alpha-fission discrimination and the time resolution were optimized. Moreover, to reduce the neutron background and spectra distortions, neutron scattering with the materials were minimized by the choice of material and structure. These chambers were then tested and validated during tests and inbeam experiments. & 2016 Elsevier B.V. All rights reserved.
Keywords: Fission chamber Alpha-fission discrimination Prompt Fission Neutron Spectra
1. Introduction Prompt Fission Neutron Spectra (PFNS), and neutron data measurements in general, are in the center of interest for nuclear reactors, global security or fundamental understanding of the fission process. As far as the PFNS are concerned, evaluated data show discrepancies on the low (below 1 MeV) and high (above 5 MeV) energy parts for different major and minor actinides. In addition, this observable is important for the understanding and modeling of the fission process. Unfortunately the experimental data are rather scarce, specifically for the high energy neutron induced fission reactions. Recent publications [1,2] point out the needs on new experimental data to produce new PFNS evaluations with uncertainties for actinide nuclei in accordance with the Coordinated Research Project (CRP) “Evaluation of Prompt Fission Neutron Spectra of Actinides” established by the IAEA Nuclear Data Section in 2009 [3]. With this mind, we decided to launch a new measurement campaign based on improved experimental tools. In our experimental approach, PFNS are measured with the time-of-flight technique between a fission chamber and neutron detectors, set at a flight distance close to one meter. The energy resolution is then related to the time resolution of the setup, and the accuracy of the data depends on the level of background in fission and neutron detectors. Our available fission chambers were built in 70's and then degraded with time. Moreover, they were not optimized for this type of measurements. In addition, we took advantage of the n
Corresponding author. E-mail address: [email protected] (B. Laurent).
http://dx.doi.org/10.1016/j.nima.2016.06.137 0168-9002/& 2016 Elsevier B.V. All rights reserved.
improvements in electronics to develop new tools. New fission chambers, dedicated to this type of studies, were then developed, to complete the development and characterization of neutron detectors to reduce the detection threshold in PFNS measurement [4]. The development of these fission chambers was focused on different points: unambiguous alpha-fission discrimination to ensure an accurate fission trigger; excellent time resolution to perform high resolution time-of-flight measurements; very low quantities of structure material to avoid neutron spectra distortion; and a reasonable amount of actinides to obtain a sufficient count rate for high statistics measurements. A version of the chamber, containing 252Cf and dedicated to calibration measurement was also built with the same characteristics. All these requirements will be detailed in the following sections.
2. Structure of the chamber The detector is a multi-layer fission chamber containing a stack of axial ionization cells. The cathodes are referenced to the ground, while the anodes are polarized to a potential which depends on the gas mixture, its pressure and the gap between plates. In order to reduce the amount of material, and therefore limit the neutron scattering, as thin as possible hydrogen-free structure materials are used for all the parts of the chamber. Figs. 1 and 2 show a picture and a sketch of the chamber. The housing is made of 1.5 mm thick aluminum tube of square section. The external dimensions are 80 × 80 × 220 mm3. The housing is closed by two aluminum flanges, supporting a 100 μm thick, 38.5 mm diameter titanium windows in order to minimize the material budget in the incoming neutrons path.
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of structural material, the possible distortion on Prompt Fission Neutron Spectra of 238U due to the chamber can be corrected using the 252Cf reference.
4. Gas The choice of the gas is of utmost importance to fulfill the detection specifications. It has to exhibit a high drift velocity and provide a good alpha-fission separation. For those reasons, we chose to use Tetrafluoromethane with the newly developed fission chambers.
Fig. 2. Sketch of the fission chamber with the main characteristics.
In the chamber, the actinide backings consist of 50 μm thick, 59 mm diameter titanium disks, held by four insulating rods made of Torlons [5] exhibiting a hydrogen content of 2%. The cathodes are connected to a common ground while the anodes are paired two by two to SMA feed-through connectors. Dedicated preamplifiers (see details in Section 5) are directly connected to the other side of these connectors.
3. Actinide deposits The studied actinides emit alpha particles and undergo fission either spontaneously or induced by a neutron absorption. Both events, alpha emission or fission, produce signals related to the energy loss in the gas of the chamber. It is crucial to discriminate between those two classes of events to trigger the acquisition system only when a fission occurs. In order to reach a good alphafission discrimination, the thickness of the deposit should be low. However this leads to a severe limitation for the total possible embedded actinide mass. In order to satisfy these two constraints, several couples of anode/cathode were stacked in the chamber, while keeping a reasonable number of electronic channels. Two fission chambers were constructed and tested. The first prototype was equipped with one cathode supporting a 25 mm diameter electro-deposit of 252Cf sample. Since 252Cf is both an alpha emitter and undergoes fission spontaneously, it is a good candidate to test the response of the fission chamber to those competing decay modes. Fission neutron detection efficiency measurements of neutron detectors were also made with this chamber, since PFNS of 252Cf is well known and recommended as a evaluated standard [6]. The second fission chamber was built to contain 238U and used on neutron beam facilities. A total of 72 deposits (33 mm diameter, 5 mg) were produced by electroplating at the CACAO [7] facility for a total of 360 mg of 238U. The uranium was deposited on both side on each anode and cathode, except on both extremities, where the deposits were only on the internal sides. Since both fission chamber (252Cf and 238U) are similar in terms
4.1. Drift velocity The time resolution is a key point in time-of-flight measurements. Since the fission chamber acts as the “start” of the timing measurement, a good intrinsic resolution is needed. When an alpha emission or a fission occurs, emitted particles ionize the gas and electron-ion pairs are created. The electrical signal on the electrodes is mainly due to the drift of electrons towards the anode. The faster the electron, the shorter the rise time of the signal will be. This is obtained choosing an ionizing gas showing a high electron drift velocity, at reasonable electric field, such as Tetrafluoromethane (CF4). Fig. 3 compares the electron drift velocity, obtained from Garfield [8] simulations, as a function of electric field for different gases usually used in fission chambers like P10 (90% Ar, 10% CH4) or P20 (80% Ar, 20% CH4), and CF4. Although CF4 requires a higher electric field to reach high drift velocity, the value obtained at 1.3 kV/cm, 11 cm/μs, is roughly a factor of two higher than that typically reached by other detectors. The CF4 gas exhibits also the advantage to show a higher density compared to most gases used in ionization chambers. Hence it allows a small gap distance between anodes and cathodes to be kept even at atmospheric pressure. Thus, we could obtain a compact, and light weight fission chamber despite a large number of plates (37). However, this gas has the disadvantage to be very sensitive to the oxygen pollution, requiring a permanent gas flux in the chamber. A regulation gas system is then used, to insure a constant flux and a pressure 50 mb above the atmospheric pressure during the measurements. Once the gas was chosen, the other parameters, such as gap between electrodes, voltage value… should be fixed accordingly to the gas specificities as described in the following section.
Drift velocity (cm/µs)
Fig. 1. Pictures of the fission chamber (outside: left and inside: right). The electronics cards are the motherboard hosting the preamplifiers (not in place). On the right side, cathodes are grounded while anodes are connected to the feed-through connectors.
12
10
8
6
CF4
4
P10
2
P20
0
0
200
400
600
800
1000
1200
1400
E field (V/cm) Fig. 3. GARFIELD simulation [9] of the electron drift velocity (cm/μs) as a function of electric field (V/cm) at atmospheric pressure for P10, P20 and CF4. The dot and dash lines represent the value of field (and corresponding drift velocity) chosen for the operation of the fission chamber.
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4.2. Stopping power
5. Electronics and acquisition system
In order to separate properly alpha decays from fission events, the electric charge induced by an alpha decay in the gas should be lower than that of fragments emitted in the fission process, for all emission angles. The induced charge qind on the anode is depending on the gap D between the anode and the cathode, the charges Q0 created by the particles and the charges path length L in the gas perpendicular to the electrodes, following the RamoShockley theorem:
5.1. Preamplifier
qind = Q 0 ×
L D
(1)
The most unfavorable case occurs when alpha particles are emitted parallel to the deposit (the induced charge correspond then to the full alpha energy ( E6 MeV which is around the maximum alpha energy for actinides)) while fragments are emitted perpendicular. In that case, assuming a constant stopping power along the trajectory (the charges are created in average in the middle of the gap: L = D/2), the fragment induced charge is then equal to Q 0/2. Thus, the fission fragment should lose at least 12 MeV, to induce a charge equivalent to the worst case alpha emission. Moreover, in case of high alpha activity, alpha pileup leads to a degradation of the separation. To limit the overlap between alphas signal and fission signal, we designed the chamber so that fission fragments show an energy loss of at least twice the maximum induced charge of an alpha. For a light fission fragment (Z¼ 36) at 120 MeV, LISE þ þ [10], with the method from [11] for energy loss calculations show that an energy loss of 24 MeV in CF4 at atmospheric pressure correspond to a path of 1.5 mm. To be conservative, and due to mechanical considerations, the gap was then chosen equal to 2.5 mm. This value is a good compromise between the discrimination performances, the stackup capacitance and the total length of the chamber. Considering Fig. 3, to reach a drift velocity of 11 cm/μs, an electric field of 1300 V/cm has to be applied between anodes and cathodes. With the chosen gap at 2.5 mm, a potential of 325 V on the anodes is applied, the cathodes being grounded. A drift velocity of 11 cm/μs means also a total electron drift duration of 23 ns, leading to a good intrinsic time resolution of the fission chamber, providing that the electronic chain does not add an extra contribution.
3
Good drift velocity in the gas is not enough to ensure a good time resolution, the electronics should be adapted to fast signals. A dedicated preamplifier was developed to meet the requirements of this fission chamber. As can be seen on Fig. 4, a specific motherboard hosting three preamplifiers has been designed. Two low voltages and one high voltage are common to the three preamplifiers. They are provided through LEMO connectors. The charge signal outputs are connected through SMA connectors while the anode signals from the fission chamber are collected on the back side of the motherboard, directly via the feed-through SMA connectors of the fission chamber. The absence of cables between the chamber and preamplifiers reduces the input capacitance and improves the signal over noise ratio. The preamplifier intrinsic rise time has to be short to preserve the good timing response of the chamber, and the duration of the signal has to be short enough to avoid excessive pileup. The choice was to develop charge preamplifiers (to ensure good amplitude discrimination) with a short RC decay time constant to avoid the saturation of the preamplifier in case of very high alpha activity. Fig. 5 shows a collection of waveforms obtained with the 252Cf chamber. The separation, in terms of amplitude, between alpha and fission signals is clearly visible. The full width at half maximum of the signal does not exceed 700 ns, and the rise time (10– 90%) is of 18 ns. For the 238U chamber, we chose to interconnect two anodes on the same preamplifier to increase the total actinide mass while keeping a reasonable number of electronic channels. 5.2. Digital acquisition system: FASTER The signals from the fission chamber preamplifier are directly sent to the digital acquisition system FASTER (Fast Acquisition SysTem for nuclEar Research), currently being developed at LPC Caen [12]. Signals are digitized by a 500 MHz, 12 bits Analog-toDigital low noise (1.1 lsb rms) Converter (ADC) and processed by real time numerical modules, implemented on FPGAs (Field Programmable Gate Array). The analog bandwidth of the sampling ADCs is limited at 100 MHz thanks to a passive input low pass filter, in order to ensure a good zero time crossing determination on the Constant Fraction Discriminator (CFD) signal (using a 2nd order polynomial interpolation), and to optimize the signal over noise ratio. Finally FASTER can provide a 7.8 ps-accuracy time data. The real time treatment is fast enough to authorize high count rate
Fig. 4. Left: picture of a motherboard hosting three preamplifiers and the different connections (signal input/output, low and high voltages). Right: back side of the same motherboard with the SMA input connectors to the fission chamber.
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Amplitude (V)
Amplitude (V)
4
0.12 0.1 0.08 0.06 0.04 0.02
0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 -0.05 -0.01
0
18 ns 0
0.01
0.02 Time (μs)
-0.02 -0.04 -0.06 -0.08 0
1
2
3
4
5
Fig. 5. Preamplifier signal from the fission chamber. Alpha (lower amplitude) and fission events are clearly visible. The inset shows the rise time of about 18 ns, compatible with the drift velocity of the gas for a 2.5 mm gap.
(up to 700,000 events per second). Only relevant parameters (CFD timing, integrated charges, pulse height etc.) are stored. A graphical user interface and an online visualization program on the acquisition computer allow to setup and monitor all experiment parameters. Moreover, by absolute time stamping of each event, the reconstruction of coincidences off-line or a complex analysis is possible.
Scattered/direct neutron
Time (μs) 0.2
0.15
0.1
0.04
6. Simulations of neutron background and prompt neutron spectra distortion When measuring prompt fission spectra, several sources of background and distortion have to be considered. The fission chamber was conceived to minimize the scattering of the beam neutrons on the apparatus and subsequently detected in the neutron detectors by random coincidences. As for fission neutron, their scattering by the chamber structure elements and body was also investigated. In order to estimate neutron scattering on the fission chamber material, Monte Carlo simulations were performed using the MCNPX [13] code. Two components were studied; the first one consisted in placing the chamber into a mono-energetic neutron beam and measuring the fraction of neutron scattered on the structure and detected around the chamber. The second one consisted in simulating a neutron source with an energy distribution which followed a Watt spectrum on one of the deposit in the center of the chamber. We estimate the distortion of this spectrum out of the chamber for different emerging angles. 6.1. Neutron beam scattering The fission chamber is developed to be used in different facilities with collimated/focussed (LICORNE at IPNO, WNR at Los Alamos, NFS at GANIL…) or non-collimated (4 MV Van de Graaff at Bruyères-le-Chatel) neutron sources. When collimated neutron beams are used, only the entrance and exit windows, together with the backing of the deposits, can contribute to neutron scattering. In the other case, the whole fission chamber scatter the neutrons. In order to simulate both configurations, a mono-energetic (from 1 to 15 MeV by 1 MeV steps) uniform cylindrical source of two different diameters is defined. The sources are represented by the cylinders in the scheme of Fig. 6 (upper part). They are characterized by a diameter of 33 mm to match the expected neutron
0.03
0.02
0
2
4
6
8
10
12
14
16
En incident (MeV) Fig. 6. Ratio of scattered neutrons over beam neutrons for a non collimated beam (upper spectrum) and a collimated beam (lower spectrum). The picture on the top part represents the scheme of the simulated configurations, with the two beam diameters and the detection surface (truncated sphere) used for scattering estimations.
beam and a diameter of 140 mm to cover the whole fission chamber. Ratios between neutron passing through the truncated sphere (in the interval 45–135°relative to the beam) and inside one of the cylinder, yielded the scattered neutrons over direct neutrons. The ratio is plotted on the lower part of Fig. 6 for different incident energies. Assuming a neutron beam flux of 106 n/cm2/s, at 2 MeV incident neutron energy (the worst case scenario in term of scattering), the fission rate is of 450 Hz for 360 mg of 238U. Since the average multiplicity of fission neutrons is 2.7 [14], the number of emitted neutron is 1215 n s 1. When the diameter of the neutron beam is 33 mm, for the same neutron flux of 106 n/cm2/s and a scattered ratio of 0.033, the number of scattered neutrons is about 2.8 × 105 per second. However, considering a coincidence windows of 200 ns after each fission event to detect the prompt fission neutron, the number of scattered neutron in the same time is then equal to 25 n s 1, compared to the 1215 emitted prompt neutrons. In the case of non-collimated neutron source, the same
J. Taieb et al. / Nuclear Instruments and Methods in Physics Research A 833 (2016) 1–7 ×10 252
Cf(sf)
70
1
Counts
80
Counts
0°
1.1
5
10
60 10
α
50
0.9
10
40
2
3
4
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6
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7
30
0
5
10
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45°
1.1
×10 25 30 35 Charge (a.u.)
20 10
1
0
fission events 0
5
10
15
20
0.9
25
×10 30
Charge (a.u.) 1
2
3
4
5
6
8
7
Fig. 8. Zoom in the ordinate axis of the pulse height spectrum of the 252Cf, showing the very good alpha-fission discrimination. The inset is the ordinate axis log scale of the same spectrum.
90°
1.1
1
0.9 1
2
3
4
5
6
7
8
En incident (MeV) Fig. 7. Ratio of the spectra simulated outside the chamber with the Watt neutron source spectrum for the three different angles. The dashed lines represent 7 5% of distortion. The lower threshold is set at 400 keV.
calculation give a number of 2610 s 1 scattered neutrons, compared to the 1215 s 1 prompt neutrons. That means that even with a dedicated fission chamber, a collimated, or focussed, neutron beam is highly advantageous to limit the neutron background in the neutron detectors, whereas extracting properly the prompt neutrons from background neutrons in non-collimated beam configuration is still challenging. 6.2. Prompt fission neutron spectrum distortion For the second type of simulations, the neutron source consists in a typical Watt spectrum placed at the center of the chamber. The energy of the neutrons detected outside the fission chamber was reconstructed from their simulated time of flight at three different angles relative to the beam direction: 0°, 45°and 90°. The ratio of these spectra to the source spectrum, is plotted on Fig. 7. The results show that the spectrum distortion is maximum at 0°. It denotes that the titanium backing (50 μm thick × 36 plates) of the deposit is preponderant in the distortion effects. Therefore, for further improvements, thinner backing (25 μm) will be considered. The prompt fission neutrons emitted in the fission chamber and flying to the neutron detectors placed between 45° and 135°, including 90°, are little affected: less than 5% of distortion between 600 keV and 2 MeV, less than 10% in the whole energy range above the threshold set at 400 keV.
spontaneously. Thus the discrimination between alpha decay and fission events could be studied without the need of a neutron beam. Fig. 8 shows the pulse height spectrum obtained with the FASTER system and illustrates the achievable discrimination. The overlap between the two components is almost null. In order to check the time resolution, a coincidence measurement between the fission chamber and a LaBr3:Ce fast gamma detector was performed. The intrinsic time resolution of the LaBr3:Ce detector was previously measured to be of 370 ps [4,15]. Fig. 9 shows the time-of-flight spectrum obtained between the start signal given by the fission chamber and the stop signal from the gamma detector. The width of the prompt gamma peak obtained represents the quadratic sum of both the LaBr3:Ce and the fission chamber resolutions. From that, a value of 732715 ps (FWHM) for the chamber coincidence peak resolution is extracted, showing its very good timing. 7.2.
238
U fission chamber
Prompt Fission Neutron Spectra measurements were performed on the LICORNE [16,17] beam line at the IPN Orsay laboratory and on the LANSCE/WNR [18,19] facility at Los Alamos, using the 238U fission chamber, in coincidence with liquid scintillator neutrons detectors array. Both anodes and cathodes were coated with a 0.6 mg/cm2 thick depleted uranium, so that the total
Counts
Ratio output/source
1
FWHMFC = 732 ps
3000 2500 2000 1500 1000 500
7. Performances measurements 7.1.
252
Cf prototype
A first fission chamber prototype containing 252Cf as a fissile material was developed and built. The alpha activity of the sample was of 65 kBq. 252Cf is both an alpha emitter and undergoes fission
0
20
20.5
21
21.5
22
22.5
γ time of flight (ns) Fig. 9. Prompt gamma peak time-of-flight spectrum between the 252Cf fission chamber signal and a LaBr3:Ce gamma detector. The fit of the peak (dotted red Gaussian) give a total FWHM of 820 ps, leading to a time resolution of the fission chamber equal to 7327 15 ps (FWHM). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
J. Taieb et al. / Nuclear Instruments and Methods in Physics Research A 833 (2016) 1–7
Counts
6
Table 1 Comparison of the main characteristics of the old and new fission chambers.
without /with incident neutron beam 238
U
1
Gas Structure material Total mass of structure material Housing thickness Beam axis thickness (windows þ backing) Neutron beam scattering Maximum spectrum distortion Timing resolution Alpha-fission discrimination
10
10
fission events
α 0
5
10
15
20
25
×10 30
Charge (a.u.)
Counts
Fig. 10. Pulse height spectra of the 238U, with a low fission event overlap. Contrary to the 252Cf, for 238U the alpha only contribution is easily obtained by running the chamber without incident neutron beam (red dashed spectrum). Counts are normalized to the run duration in order to compare easily both spectra. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
anodes and cathodes coated
10
only cathodes coated 10
10
10
5
10
15
20
25
× 10 30
Charge (a.u.) Fig. 11. Comparison of the alpha-fission discrimination: deposits on anodes and cathodes (full line) and deposits on cathodes only (dashed line).
amount of fissioning material was 360 mg. Two anodes are connected to the same preamplifier, meaning that 8 deposits are summed on one acquisition channel. Due to the low alpha emission, the pileup and count rate remain acceptable. The chamber performances obtained, in terms of alpha-fission discrimination, are very similar to that of the 252Cf one, as shown in Fig. 10. The overlap between alpha and fission events is higher than that of the 252Cf, due to the uranium deposit thickness, but it is still negligible. During the development of the chamber, tests with deposits on cathodes only were performed. The discrimination is even better in this case, as shown in Fig. 11. This configuration could be used in measurements where the number of fissions has to be known very precisely, to the detriment of the amount of actinide, and therefore of the fission count rate. In the case of Prompt Fission Neutron Spectra measurements, the statistics recorded is crucial and the configuration with deposits on both anodes and cathodes were used, using a slightly higher threshold in order to eliminate alpha and non separated fission events.
8. Conclusions New fission chambers were developed to optimize the Prompt Fission Neutron Spectra measurements, to lower the distortion of detected neutron spectra and to improve the time resolution and
Old FC
New FC
P20 (6 b) Stainless steel 4 kg 5 mm 12 mm 30% 20% 5 ns 20%
CF4 (1 b) Aluminum 1 kg 1.5 mm 2 mm 3% 5% 1 ns <2%
alpha-fission discrimination. Not only the fission chamber itself but also the whole experimental system was optimized to achieve very good performances. The Table 1 shows the improvement of the main characteristics of this new fission chamber, compared to our typical old fission chamber. These obtained characteristics allow to perform new Prompt Fission Neutron Spectra measurements with better statistics and better energy resolution. This work showed that with thinner deposits on cathodes only, it is possible to separate completely alpha and fission (see 252Cf tests in Section 7.1). Other experiments, where the number of fission has to be known exactly can be investigate with such a chamber: (n, xn) measurements with a veto fission or fission cross section measurements. However, the work on the fission detection efficiency has to be completed to extract accurate data. The good characteristics of the fission chamber can be also useful for Prompt fission γ-rays spectra measurements, which are very sensitive to distortion on setup material. Specific tests with high activity samples (up to 14 MBq alpha activity) have been performed to estimate the alpha fission discrimination reached in such configuration, and improvements have been made to increase this efficiency. Results will be presented in a forthcoming publication. A fission chamber containing a total amount of 50 mg of 239Pu is planed to be assembled. We intend to use that chamber to perform Prompt Fission Neutron Spectra measurement with high statistics, good time resolution and alpha-fission discrimination capability.
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