|Year : 2019 | Volume
| Issue : 4 | Page : 138-143
Copper activation detector for high energy-pulsed neutron measurements
Priyamvada M Dighe
Electronics Division, Bhabha Atomic Research Centre, Trombay; Homi Bhabha National Institute Anushakti Nagar, Mumbai, India
|Date of Submission||11-Oct-2019|
|Date of Decision||27-Nov-2019|
|Date of Acceptance||03-Dec-2019|
|Date of Web Publication||27-Jan-2020|
Dr. Priyamvada M Dighe
Electronics Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085, Maharashtra
Source of Support: None, Conflict of Interest: None
A first of its kind threshold activation proportional counter is developed using copper for the measurement of pulsed 14 MeV neutrons. The detector is of 26 mm outer diameter and 200 mm overall length. Tests at D-T facility showed that the detector is capable of measuring high-energy neutrons. The high-energy neutron sensitivity of the detector is estimated as 7.6 × 10−5 counts/neutrons cm-2/s, and pulsed neutron sensitivity is estimated as 3.3 × 107 neutrons pulse−1 count−1. The low-energy response of the detector can be discriminated by curve fitting method to the decay counts. An identical dimension silver-lined proportional counter with 0.5 counts/neutrons cm-2/s thermal neutron sensitivity is developed for comparison with the copper detector. The silver activation detectors are conventionally used for neutron yield measurement of D-T-pulsed source. Experiments confirmed that compared to silver detector which is sensitive to low-energy neutrons, the copper detector is able to measure high-energy neutrons directly by threshold reaction.
Keywords: 14 MeV neutrons, pulsed activation detector, threshold reaction
|How to cite this article:|
Dighe PM. Copper activation detector for high energy-pulsed neutron measurements. Radiat Prot Environ 2019;42:138-43
| Introduction|| |
Measurement of pulsed 14 MeV neutrons is becoming important for several applications, such as D-T neutron generators, high-energy accelerators, and spallation sources. Conventional high-energy neutron detectors utilize one of the following techniques: the proton recoil detectors, the fission chambers, and the neutron reactions withl0 B,3 He, and7 Li. Proton recoil scintillators suffer much interference from gamma rays, electrons, and also from lower energy neutrons, due to the continuous nature of their response. Fission detectors are also unsuitable for 14 MeV measurements as threshold response is about 1.5 MeV. Thel0 B,3 He, and7 Li neutron reactions are very sensitive to thermal neutrons. Foil activation techniques utilizing threshold reactions are commonly used for measurement of high-energy (14 MeV) neutrons. In foil activation method, a suitable material is exposed to a neutron source. The induced activity is measured, and then, knowing the activity induced per neutron incident on the sample, the total neutron yield is estimated off-line. The off-line measurement has an obvious disadvantage of long interval time between the measurements. For on-line measurement of pulsed neutrons, detectors based on silver activation are used., Silver is thermal neutron detector, and hence, 14 MeV neutrons are measured indirectly through calibration. For on-line measurement of pulsed 14 MeV neutrons, a first of its kind, a threshold activation proportional counter using copper as activation material is developed and calibrated for 14 MeV neutrons. The detector has a cylindrical shape with 26 mm overall diameter, and 200 mm overall length, is mechanically rugged, has an all-welded design. A mechanically identical silver activation proportional counter is also developed for comparison with the copper detector. The present article describes the design, development, and calibration of copper activation detector.
| Principle|| |
Threshold activation counter
Threshold reactions in nuclear activation techniques are traditionally been used for measurement of high-energy neutrons. Copper is largely used to measure the 14 MeV neutrons via the reaction:63 Cu (n, 2n)62 Cu(β+). This reaction has a threshold of 11 MeV as shown in [Figure 1]. A proportional counter using copper as converter material is developed for and tested for performance for high-energy neutrons. The interaction of neutrons with copper eventually ionizes the filled gas and produce signal proportional to the incident neutron flux.
|Figure 1: Threshold reaction of63Cu (n, 2n)62 cross-section plot (ENDF/B-VII.1)|
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Natural copper has two stable isotopes63 Cu (69.17%) and65 Cu (30.83%). Neutrons interact with both the isotopes via n, 2n and n,γ reactions. The threshold reaction of63 Cu (n, 2n)62 Cu (p+) has decay half-life of 9.74 min and is primarily important. The n,γ reaction with63 Cu and n, 2n reaction with65 Cu has decay half-lives of 12.7 h which is very large and do not contribute significantly for short duration-pulsed neutron irradiations. However, the n,γ reactions of65 Cu has 5.12 min decay half-life which can interfere with the high-energy neutron signal. Therefore, for estimation of threshold reaction response of the copper detector for 9.74 min half-life, an analytical technique of curve fitting is evolved to discriminate the signal produced for 5.12 min half-life from low-energy neutrons.
Silver activation counter
An identical dimension proportional counter with silver as converter material is developed. Natural silver has two stable isotopes:109 Ag (51.8%) and107 Ag (48.2%) which undergo n,γ reactions with neutrons with decay half-lives 24.6 s and 142.2 s, respectively. The radiations emitted in neutron reactions ionize the filled gas and produce a signal which decays with time. These decay data are used for calibration of continuous and pulsed neutron sources.
| Design|| |
[Table 1] gives the main specifications of the detectors developed. The outer dimensions of the detectors were chosen to fit in a standard neutron flux monitors. [Figure 2] and [Figure 3] show the pictures of the detectors and components, respectively. Neutron sensing copper and silver is introduced in the form of baffles in the detector to increase the surface area of activation material which resulted in enhanced sensitivity. The detectors' outer body is made out of SS 304 L. Ceramic-to-metal feedthroughs are used as insulators. 25 μm diameter tungsten anode wire is mounted axially using suitable spring assemblies. The outer body together with baffles act as cathode. The detectors are completely welded, and weld joints are leak tested before gas filling. The detectors were outgassed and filled with P10 gas (Ar - 90% + Methane - 10%) at 30 cm of Hg pressure.
|Figure 2: Picture of threshold (copper) activation and low-energy (silver) activation proportional counters|
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| TESTS and RESULTS|| |
The copper and silver detectors were tested for performance at the D-T neutron source facility of BARC. The D-T neutron source is accelerator based. In our experiments, neutron yields of ~107–108 neutrons/s were used. The detectors were placed horizontally in front of the D-T neutron source, as shown in [Figure 4]. Since the neutron source is isotropic, the number of neutrons falling on the detectors was estimated using solid angle calculations for a cylindrical shape detector. The detectors were connected to HV supply, charge-sensitive preamplifier, and shaping amplifier. The data were acquired digitally using a multichannel analyzer device operating in multichannel scaler (MCS) mode.
|Figure 4: Experiment setup of the detectors at 14 MeV neutron source facility|
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Functional tests of the detectors
Tests were conducted by placing the copper detector at variable distances from the D-T source and also by varying neutron yields. The detector was operated at 1050 V HV bias, and the discriminator bias was set above electronic noise. The detector was placed at 3.8 cm distance from the source surface and was irradiated with 5.6 × 107 neutrons/s yield for 9.74 min duration, and then, the source was switched off. While irradiation, buildup of copper activity was observed. After the D-T beam was switched off, the copper activity started decaying following the decay half-lives of the copper. The decay data were acquired in MCS mode with 1s bin width. The decay data of activation detectors are derived by following equation A.
Where, C are counts measured at a given time t, W is weightage which corresponds to the incident flux φ, efficiency ε, n = number of decay constants, B = Background count rate, N is no. of atoms, σ is microscopic cross section, λ is decay constant, and tirr = time of detector irradiation.
The decay data were analyzed using ROOT minuit software and were fitted with equation A. The detector irradiation was repeated for 9.74 min duration by covering the detector with 2.54 cm thick high-density polyethylene (HDPE) cylindrical sleeve and again decay data were acquired with 1s bin width. The data for both the irradiations were first fitted with one decay constant and then with sum of two decay constants. [Figure 5] and [Figure 6] give the decay data with fit for the irradiation in bare and HDPE-covered conditions, respectively, with fit for one decay constant and two decay constants. [Table 2] gives the fit parameters obtained.
|Figure 5: Decay data fit of the copper detector in bare condition with one decay constant and with two decay constants|
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|Figure 6: Decay data fit of the copper detector in high-density polyethylene covered condition with one decay constant and with two decay constants|
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The experiment was repeated by placing the silver detector at 3.8 cm distance from the source surface and was irradiated with 5.6 × 107 neutrons/s yield for 120 s. Similar to the copper detector, the silver detector was also irradiated first in bare condition and then covered with 2.54 cm thick HDPE sleeve. The decay data were acquired after the source was switched off and was fitted with the two decay constants of silver using equation A. [Figure 7] gives the decay curves of the silver in both the conditions.
|Figure 7: Decay data fit of the silver detector irradiated in bare and covered with high-density polyethylene conditions|
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| Discussion|| |
It is well understood that 2.54 cm thick HDPE cannot moderate 14 MeV neutrons. However, a significant rise (ten times) in the silver detector signal after covering with the 2.54 cm HDPE sleeve shows the presence of scattered intermediate energy range neutrons which are moderated by the HDPE. When the copper detector is covered with 2.54 cm thick HDPE sleeve, the detector signal is enhanced by only 10 %, which shows that the low energy neutrons are producing negligible signal and high-energy response of the copper detector is predominant when used for 14 MeV neutron source. The decay data analysis as given in [Table 2] shows that when low-energy neutrons are present, curve fit with two decay constants correctly derive the threshold reaction decay half-life and therefore can be used to estimate fast neutron response of the detector in mixed neutron energy fields. However, when used with 14 MeV neutron source without moderator, the low-energy neutrons' presence is negligible compared to high-energy neutrons, and one decay constant of n, 2n reaction is sufficient to fit the decay data. In other words, in this situation, the decay data are mainly produced by high-energy threshold reactions.
Estimation of neutron sensitivity of the copper detector
The neutron sensitivity (intrinsic efficiency) of the copper detector was evaluated for continuous D-T source. The detector was irradiated with D-T source in bare condition by placing at varying distances from the D-T source, from 2.8 cm to 4.5 cm distances. The detector was irradiated for 60-s duration, and decay data were acquired after switching off the neutron beam. The data were analyzed using ROOT MINUIT to derive the counts contributed for 9.74 min decay half-life. For sensitivity calculations, the neutron flux incident on the detector was estimated using solid angle. [Table 3] gives the evaluated solid angle and neutron sensitivities at various distances and source yield. The average sensitivity of the detector is estimated as 7.6 × 10−5 counts/neutrons cm-2/s.
|Table 3: Sensitivity estimation of the threshold activation proportional counter|
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Pulsed neutron sensitivity of copper and silver detectors
In the absence of a pulsed neutron source, the neutron sensitivity of activation detectors can be estimated using a continuous neutron source by source removal method. In this method, the detector is irradiated by continuous neutron source of known yield up to saturation activity, and then, the source is removed and neutron count rate is recorded. The neutron sensitivity/calibration constant K is derived as follows: K, the calibration constant, is the ratio of the neutron source yield and the count rate recorded, Io (neutrons/sec) is the yield of radioactive neutron source; the calibration factor is estimated using the following equation:
Here, NCR are the saturated counts measured after the neutron source is removed or switched; λ is the decay constant of the activation counter material, in this case copper or silver.
Using the saturation decay count rate data of the bare copper and silver detectors, the pulsed neutron sensitivity for 14 MeV neutrons of the copper detector is estimated as 3.3 × 107 neutrons pulse−1 count−1, and for silver detector, it is estimated as 2.2 × 107 neutrons pulse−1 count−1.
Thermal neutron sensitivity of silver detector
The silver detector is thermal neutron detector. Its thermal neutron sensitivity was measured in thermal neutron stag facility of RSSD, BARC. The detector was placed in thermal neutron flux of 150 nv (neutrons cm2/s). [Figure 8] gives the detector count rate with respect to operating voltage. We have selected 1050 V for all our experiments. Neutron sensitivity is estimated as 0.5 cps/nv.
| Conclusions|| |
The threshold activation proportional counter is developed using copper as neutron-sensing material. Tests with 14 MeV neutron source showed that the detector is sensitive to high-energy neutrons, and the threshold reaction response can be derived by curve fitting method, and its sensitivity to high-energy neutrons is estimated as 7.6 × 10−5 counts/neutrons cm-2/s. Its pulsed neutron sensitivity is derived as 3.3 × 107 neutrons pulse−1 count−1. The detector can measure high-energy neutrons directly and is a good substitute to foil activation measurements where in situ results are required. Although silver counter is conventionally used for yield measurement of pulsed D-T sources, it is actually sensitive to low-energy neutrons; hence, scatter component can affect the calibration factor significantly. The copper detector showed lower sensitivity to low-energy neutrons produced by scattering. The low-energy response of the copper detector can be discriminated using curve fitting algorithm to the decay data. However, copper detector has limitations due to its long decay half-lives, and therefore, short irradiation periods and sufficient cooling periods between measurements are required for pulsed neutron yield calibrations.
The author is grateful to Mrs. Anita Behere, Head, Electronics Division, BARC and Mr. P.V. Bhatnagar, Head Reactor Instrumentation Section, Electronics Division, BARC for their constant support and encouragements. Special thanks to Mrs. Lata P. Kamble, Electronics Division, BARC for assembling the detectors and assisting with the experiments.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]
[Table 1], [Table 2], [Table 3]