Thermal Neutron Monitor Research Articles

Advances in acoustically-tensioned metastable detector neutron detection technology

Nuclear security, the prevention of special nuclear material (SNM) smuggling, at borders and ports has been described as one of the 21st century grand challenges. Currently, thermal neutron detectors (a majority of which are He-3 detectors), are used to monitor for neutrons emitted from special nuclear materials. He-3 neutron absorption cross-sections are ∼103 times higher for thermal vs fast neutrons; as such, moderated He-3 detectors must suffer from reduced efficiency (due to scattering and absorption in moderators) for detection of fast neutrons emitted from fission of SNMs. Development of a real-time high-efficiency gamma/beta blind, directionality-determining thermal and fast neutron monitoring system without need for bulky moderation is needed to allow for optimal SNM monitoring. The previously developed economical acoustically tensioned metastable fluid detector (E-ATMFD(Ver.0), offered all of these capabilities but exhibited low (∼0.01–0.05%) efficiencies than conventional neutron monitors. Design optimization guided via coupled multiphysics-neutron transport simulation tools, together with devising a figure of merit algorithm, experimental validation work with various reflector shapes, and a novel detection fluid “DFP”, the direct drive E-ATMFD(Ver.1) architecture was devised and demonstrated to offer ∼5 to 27x improvement in neutron detection efficiency. Using borated DFP mixture version E-ATMFD(Ver.1B), epithermal to fast neutron detection capabilities were demonstrated — effectively extending the sensitivity to all energies of neutrons and increasing the efficiency by another factor of up to ∼2.5. Further research included development of a novel frequency sweep-cum-rate drive mode to successfully operate multiple E-ATMFDs simultaneously from a single power source; resulting in a further 4x detection efficiency improvement from enhanced scattering and electromagnetic coupling — showing potential for enabling large array mode SNM interrogation.

A study on the optimization of processing parameters of boron-doped CR-39 solid-state nuclear track detectors for response to thermal neutrons

The methodology for personnel monitoring of neutrons in the intermediate and fast energy range using CR-39 solid-state nuclear track detectors (SSNTDs) is well established. This study presents the dosimetric response of boron-doped (carborane) CR-39 detector for the measurement of thermal neutrons. It includes optimization of etching process, minimum detection limit (MDL), and thermal neutron sensitivity. Duration of low frequency (100 Hz) and high frequency (3.5 kHz) for elevated temperature electrochemical etching was optimized as 4 h and 50 minutes respectively, and used for the response characteristic study. The measured thermal neutron sensitivity and MDL were 743 ± 13 tracks cm−2 mSv−1 and 0.02 mSv, respectively. This study is useful for thermal neutron monitoring in reactor, accelerator, and all other nuclear facility environment.

Real-time detection of fast and thermal neutrons in radiotherapy with CMOS sensors

The peripheral dose distribution is a growing concern for the improvement of new external radiation modalities. Secondary particles, especially photo-neutrons produced by the accelerator, irradiate the patient more than tens of centimeters away from the tumor volume. However the out-of-field dose is still not estimated accurately by the treatment planning softwares. This study demonstrates the possibility of using a specially designed CMOS sensor for fast and thermal neutron monitoring in radiotherapy. The 14 microns-thick sensitive layer and the integrated electronic chain of the CMOS are particularly suitable for real-time measurements in γ/n mixed fields. An experimental field size dependency of the fast neutron production rate, supported by Monte Carlo simulations and CR-39 data, has been observed. This dependency points out the potential benefits of a real-time monitoring of fast and thermal neutron during beam intensity modulated radiation therapies.

Early clinical experience utilizing scintillator with optical fiber (SOF) detector in clinical boron neutron capture therapy: its issues and solutions.

BackgroundReal-time measurement of thermal neutrons in the tumor region is essential for proper evaluation of the absorbed dose in boron neutron capture therapy (BNCT) treatment. The gold wire activation method has been routinely used to measure the neutron flux distribution in BNCT irradiation, but a real-time measurement using gold wire is not possible. To overcome this issue, the scintillator with optical fiber (SOF) detector has been developed. The purpose of this study is to demonstrate the feasibility of the SOF detector as a real-time thermal neutron monitor in clinical BNCT treatment and also to report issues in the use of SOF detectors in clinical practice and their solutions.Material and methodsClinical measurements using the SOF detector were carried out in 16 BNCT clinical trial patients from December 2002 until end of 2006 at the Japanese Atomic Energy Agency (JAEA) and Kyoto University Research Reactor Institute (KURRI).ResultsThe SOF detector worked effectively as a real-time thermal neutron monitor. The neutron fluence obtained by the gold wire activation method was found to differ from that obtained by the SOF detector. The neutron fluence obtained by the SOF detector was in better agreement with the expected fluence than with gold wire activation. The estimation error for the SOF detector was small in comparison to the gold wire measurement. In addition, real-time monitoring suggested that the neutron flux distribution and intensity at the region of interest (ROI) may vary due to the reactor condition, patient motion and dislocation of the SOF detector.ConclusionClinical measurements using the SOF detector to measure thermal neutron flux during BNCT confirmed that SOF detectors are effective as a real-time thermal neutron monitor. To minimize the estimation error due to the displacement of the SOF probe during treatment, a loop-type SOF probe was developed.

Application of an ultraminiature thermal neutron monitor for irradiation field study of accelerator-based neutron capture therapy.

Phantom experiments to evaluate thermal neutron flux distribution were performed using the Scintillator with Optical Fiber (SOF) detector, which was developed as a thermal neutron monitor during boron neutron capture therapy (BNCT) irradiation. Compared with the gold wire activation method and Monte Carlo N-particle (MCNP) calculations, it was confirmed that the SOF detector is capable of measuring thermal neutron flux as low as 105 n/cm2/s with sufficient accuracy. The SOF detector will be useful for phantom experiments with BNCT neutron fields from low-current accelerator-based neutron sources.

Gadolinium pyrosilicate single crystals for gamma ray and thermal neutron monitoring

Abstract Bulk Gd 2 Si 2 O 7 :Ce (GPS:Ce) single crystals obtained by Czochralski method demonstrate a high light output at γ-irradiation (3.8 times higher in comparison with Bi 4 Ge 3 O 12 (BGO)), energy resolution 13% (137Cs, 662 KeV), fast decay time (41.7 ns), and good thermal stability of light output (up to 425 K). This combination of characteristics makes this scintillator very attractive for medical imaging and high-temperature applications. Light output at thermal neutron monitoring is evaluated as twice higher in comparison with Gd 2 SiO 5 :Ce (GSO). The observed rather high afterglow level (0.2% after 20 ms) and moderate energy resolution (13%) certifies a room for improvement of these parameters by further optimization of crystal quality.

Fission reactor flux monitors based on single‐crystal CVD diamond films

AbstractDiamond based thermal neutron flux monitors have been fabricated using single crystal diamond films, grown by chemical vapour deposition. A 3 μm thick 6LiF layer was thermally evaporated on the detector surface as a converting material for thermal neutron monitoring via the 6Li(n, α) T nuclear reaction. The detectors were tested in a fission nuclear reactor. One of them was positioned 80 cm above the core mid‐plane, where the neutron flux is 2.2 × 109 neutrons/cm2 s at 1 MW resulting in a device count rate of about 150000 cps. Good stability and reproducibility of the device output were proved over the whole reactor power range (up to 1 MW). During the irradiation, several pulse height spectra were recorded, in which both products of the 6Li(n, α) T reaction, e.g. 2.73 MeV tritium and the 2.06 MeV α, were clearly identified, thus excluding a degradation of the detector response. A comparison with a reference fission chamber monitor pointed out a limitation of the adopted readout electronics at high count rates, due to multiple pile‐up processes. However, once this effect is properly accounted for, a good linearity of the diamond flux monitor response is observed as a function of the fission chamber one, as well as an excellent agreement between the temporal behaviour of the two detector response. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Development of a wide-range paired scintillator with optical fiber neutron monitor for BNCT irradiation field study

A wide range thermal neutron detector was developed based on the Scintillator with Optical Fiber (SOF) detector which has been previously used for thermal neutron monitoring during boron neutron capture therapy irradiation. With this new detector system we intended to address the issues of real-time thermal neutron flux measurement and the simultaneous measurement of a wide range of thermal neutron flux in a BNCT irradiation field which were difficult to implement with the gold wire activation method. The dynamic range of linearity of the SOF detector was expanded by using a plastic scintillator with a rapid decay time. On the other hand, the contribution of gamma rays and fast neutrons in the measured signals were compensated from those obtained by a pair of SOF detectors, one with a 6LiF neutron converter and the other without. The discrimination level for the measured signals was also optimized to further reduce the contribution of gamma rays and fast neutrons signals. A non-paralyzable system model was applied to correct for the dead-time in the detector system. A good agreement between the thermal neutron flux measured by the gold wire activation method and the paired SOF detector system was observed. However, measurements which would normally take a few days to perform with the gold wire activation method were obtained in just about 15 min using the SOF detector system. We also confirmed the dynamic range of linearity for the SOF detector system to be in the order of magnitude of 10 4.

Development of real-time thermal neutron monitor using boron-loaded plastic scintillator with optical fiber for boron neutron capture therapy

A new thermal neutron monitor for boron neutron capture therapy was developed in this study. We called this monitor equipped boron-loaded plastic scintillator that uses optical fiber for signal transmission as an [scintillator with optical fiber] SOF detector. A water phantom experiment was performed to verify how the SOF detector compared with conventional method of measuring thermal neutron fluence. Measurements with a single SOF detector yielded indistinguishable signals for thermal neutrons and gamma rays. To account for the gamma ray contribution in the signal recorded by the SOF detector, a paired SOF detector system was employed. This was composed of an SOF detector with boron-loaded scintillator and an SOF detector with a boron-free scintillator. The difference between the recorded counts of these paired SOF detectors was used as the measure of the gamma ray contribution in the measured neutron fluence. The paired SOF detectors were ascertained to be effective in measuring thermal neutron flux in the range above 10 6 (n/cm 2/s). Clinical trials using paired SOF to measure thermal neutron flux during therapy confirmed that paired SOF detectors were effective as a real-time thermal neutron flux monitor.

Detection of 236U: a possible 100-million year neutron flux integrator

It is anticipated that accelerator mass spectrometry (AMS) assay of actinides, in particular 236U and 233U in uranium and 229Th and 230Th in thorium, will become valuable for identifying the previous human use of such materials. A discussion is included concerning the possibility of using 236U as a thermal neutron monitor having an integration period of up to 100 million years using AMS to fragment all molecular interferences. Using the detection of 236U 238U at levels below 10 −13 as a design goal, a description is provided of the geometry of a proposed broad-range precision AMS spectrometer that includes a 3 MV terminal tandem accelerator and a terminal gas stripper. The elimination of interfering molecular ambiguities and backgrounds is discussed. A post-acceleration stripping procedure for M Q ambiguity elimination is presented. A novel acceleration and fragmentation protocol is also described where molecular fragmentation is carried out at ground, outside of the tandem, rather than by stripper fragmentation at high voltage. It is shown that this new procedure simplifies AMS especially for heavy elements and leads to the possibility of using smaller tandems and analysis components. Multiple scattering during terminal charge changing is considered together with its effects upon accuracy and efficiency.

Trials to Induce Neutron Emission from a Titanium-Deuterium System

On April 26, 1989, the Frascati Research Centre reported the emission of >5000 n/s over many hours from a titanium-deuterium system during changes of temperature and pressure. It was attempted to reproduce these observations using a highly sensitive (10%) thermal neutron monitor and two differently degassed titanium samples. During the 20-day experimental period, the neutron emission from these titanium-deuterium systems never exceeded 1.7 (first 8 days) and 0.6 n/s (last 12 days), averaged over 100-min intervals.

Thermal Neutron Monitoring Using Cellulose Nitrate Track Detector by Means of Spark Counter

Cellulose nitrate films were examined for thermal neutron monitoring in contact with several kinds of (n, α) converter materials such as a plastic sheet doped with 1% boron, a solidified boron oxide plate, a single crystal plate of boron oxide and that of lithium fluoride. After thermal neutron irradiation in a research reactor, the films were etched in alkaline solution. The etch-pits on the films were counted with a spark counter. The single crystal converter of lithium fluoride was found to be the most sensitive for thermal neutron. A method for producing the single crystal converter was described. Linear relation between thermal neutron fluence and spark counts was found to be in the fluence range of 2 × 10−6–3 × 107n/cm2 in case of using a lithium fluoride single crystal converter.

Thermal neutron monitoring using cellulose nitrate track detector by means of spark counter.

Cellulose nitrate films were examined for thermal neutron monitoring in contact with several kinds of (n, α) converter materials such as a plastic sheet doped with 1% boron, a solidified boron oxide plate, a single crystal plate of boron oxide and that of lithium fluoride. After thermal neutron irradiation in a research reactor, the films were etched in alkaline solution. The etch-pits on the films were counted with a spark counter. The single crystal converter of lithium fluoride was found to be the most sensitive for thermal neutron. A method for producing the single crystal converter was described. Linear relation between thermal neutron fluence and spark counts was found to be in the fluence range of 2 × 10−6–3 × 107n/cm2 in case of using a lithium fluoride single crystal converter.

Back-scatter and building-up effects on the thermoluminescence sensitivity of a thermal neutron monitor

The thermal neutron monitoring system used with ordinary thermoluminescence dosimeters has been studied. In this monitoring system, we used the capture gamma-rays from the reaction between cadmium and thermal neutrons. The effects of the back-scatter and building-up of radiation have been studied for approaching the n/γ ratio of the system to that of the mixed field. It has been found that the ratio of a system using these effects is greater than that of one without these effects, and that the minimum measurable fluence is about 10 6 n/cm 2. This method can be used for all kinds of thermoluminescence dosimeter phosphors.

A thermal and fast neutron monitor for use in a subcritical facility

A convenient mounting for a subcritical facility neutron monitor using a silicon detector is described. Slow and fast neutrons are detected by the 6Li(n,α) 3H reaction and by proton recoil, respectively.

505. 円筒燃料内照射によるラジオアイソトープの製造

The neutron flux and energy distribution inside the cylindrical fuel of JRR-2 used for irradiation have been measured by activation method, and the results compared with those obtained by irradiation at the same position without the fuel.Threshold detectors such as nickel and sulphur were used to measure the fast neutron flux. Gold and cobalt metal foils were used as thermal neutron monitor. In order to measure the contribution of resonance neutrons to the effective activation cross section, elements possessing different resonance neutron absorption energies were irradiated along with the flux monitors.It was estimated that approximately 30% of the thermal neutrons were converted to fast neutrons inside the cylindrical fuel and that the fast neutron flux was 6 times as high as obtained in the absence of fuel material, but that the epithermal index was found unchanged.