K. Sakasai

Detection of Fast Neutron by Storage Phosphors With Low Gamma-Ray Sensitivity

Storage phosphors such as SrBPO5:Eu2+ with low gamma-ray sensitivity were applied to fast neutron detection using polyethylene (PE) sheets as a proton generator that were set in front of the phosphors. The photostimulated luminescence (PSL) yields were measured by changing PE thickness when neutron energy was 5 MeV. The experimental results were compared with those by Monte Carlo simulation using PHITS code. The PSL yields by gamma-ray associated with fast neutron fields were estimated for both SrBPO5:Eu2+ sheets and commercial available imaging plates. The results showed that the imaging methods using such storage phosphors with low gamma-ray sensitivity had negligible gamma-ray influences and will be used for fast neutron imaging.

Simulation program for multiwire-type two-dimensional neutron detector with individual readout

We are currently developing a multiwire-type two-dimensional neutron detector system for use at the Materials and Life Science Experimental Facility at J-PARC. This system can attain a high response time and a high spatial resolution using the individual line readout method, and the performance parameters of the system strongly depend on a gas composition and pressure of a fill gas that is a gas mixture of He-3 and CF4. A simulation program for our system was developed to determine this gas condition. In addition, a small detector prototype was fabricated to evaluate the simulation program. The program involves the following calculations for each gas condition: the probability of a reaction between a neutron and He-3, the ranges of secondary particles generated by the nuclear reaction, the ejection angle of the particles, the wall effect caused by the conversion gap, and the pitch of the multiwire element. The simulation results agreed well with the experimental results obtained using the small detector prototype. Thus, it was confirmed that the simulation program can be effectively used to determine the gas composition and pressure when the required spatial resolution and detection efficiency are known.

Observation of neutron-induced signals using two-dimensional micro-pixel gas chamber

A gas-based neutron detection system which reads out individual channels with a micro-pixel detector element was constructed, and preliminary neutron irradiation tests were conducted. The detection system consisted of a micro-pixel detector element, a gas chamber that had feedthroughs for lines of 541 channels, amplifier-shaper-discriminator boards, position encoders with field programmable gate arrays, and a device capable of fast data acquisition. The micro-pixel detector element pulses had a short duration, the full width at half maximum of the pulse being 160 ns. Its gas gain was about 50 with an anode-cathode voltage of 560 V and a gas pressure of 0.5 MPa (0.45 MPa for He and 0.05 MPa for CF4). Signal pulse peaks of neutrons could be identified. The neutron detection system could easily distinguish neutron signals from other signals arising from electronic noise, gamma events, etc. The gas gain of the micro-pixel detector element did not change for 240 min in our irradiation test.

Readout characteristics of an imaging plate by fast laser pulses

The readout characteristics of an imaging plate (IP) by fast laser pulses have been studied using an alpha-ray source. The total photostimulated luminescence (PSL) detected with the system was proportional to the alpha-ray irradiation time. It can be used for radiation dose measurement. The accumulated radiation information in the IP is decreased by reading PSL signal with the laser pulses. The PSL decay curve in each frequency is well fitted by a single exponential function. Linear dependency of the decay constant on the repeated frequency of the pulse was observed in a frequency range from 60 kHz to 200 kHz. The readout method with fast laser pulses will be useful for development of novel radiation detection system and investigation of successive repeated readout characteristics and fading effects of the IP.

Development of a ZnS/10B2O3 scintillator with low-afterglow phosphor

A low-afterglow, 10B-doped neutron-sensitive ZnS/10B2O3 scintillator was developed. The developed ZnS phosphor has a primary decay time constant of about 60 ns with a low afterglow. The developed scintillator exhibited a mean afterglow height of 4% relative to the peak at 1 μs after the peak, which is half that of a commercial ZnS/6LiF scintillator manufactured by Applied Scintillation Technologies. The count-rate capability of a wavelength-shifting-fibre-based detector was increased to 30,000 cps by implementing the developed scintillator from 5,000 cps with the commercial scintillator.

A wavelength-shifting-fibre-based scintillator neutron detector implemented with the median point calculation method

Median point calculation (MPC) method for scintillator neutron detectors operating in photon-counting mode has been developed. The method was applied to a wavelength-shifting (WLS)-fibre-based neutron detector in order to improve its pixel size and spatial resolution. The original detector was modified for a demonstration experiment performed using the SENJU neutron diffraction instrument at J-PARC (Japan Proton Accelerator Research Complex). The detector head comprised 64 × 2 WLS fibres arranged in the x and y directions at a pitch of 4 mm. The WLS fibres were sandwiched between two ZnS/10B2O3 scintillator screens. The developed MPC method used the number of photons detected by each WLS fibre as a weight when calculating the positions of neutrons. The test detector implemented with the MPC method demonstrated that the effective pixel size and the spatial resolution were improved to 2 mm, corresponding to half the pitch of the WLS fibres.

Storage characteristics of KBr:Eu 2+ phosphors with radiators by irradiation of fast neutrons

Storage characteristics of KBr:Eu2+ phosphor were investigated when the phosphor was irradiated by fast neutron with energy of 14.8 MeV. To improve the fast neutron sensitivity, polyethylene (PE) sheets as a proton generator were set in front of the phosphors. The maximum photostimulated luminescence (PSL) yields were obtained at a PE thickness of about 3 mm. These experimental results well agreed with those by Monte Carlo simulation using PHITS code. Estimated PSL yields by gamma-ray associated with fast neutron fields were negligible. This combination use of such phosphors and PE sheets will be usable for fast neutron monitoring or imaging in a high gammaray field.

High detection efficiency ZnS scintillator for a fiber-coded linear neutron detector for thermal neutron scattering instruments

High detector efficiency ZnS/⁶LiF scintillator and ZnS/¹⁰B<inf>2</inf>O<inf>3</inf> ceramic scintillator were developed for a fiber-coded linear neutron detector for thermal neutron detection. Detector performances of the detector equipped with such scintillator strips were evaluated in terms of neutron detector efficiency, gamma sensitivity, and quiet count rate. The detector equipped with the ZnS/⁶LiF scintillator that contained twice amount of ⁶Li compared to the standard scintillator (ZnS/⁶LiF scintillator produced by AST Co Ltd) exhibited about 30% higher detector efficiency for 1-Å neutrons. The fiber-coded detector equipped with the ZnS/¹⁰B<inf>2</inf>O<inf>3</inf> ceramic scintillator also exhibited about 30% larger neutron counts than the detector made with the standard scintillator. Moreover, the scintillation light of the ZnS/¹⁰B<inf>2</inf>O<inf>3</inf> ceramic scintillator exhibited 40% shorter decay time, indicating a potential for higher count rate capability.

Gamma-ray dosimetry system using radiation-resistant optical fibers and a luminescent material

Gamma-ray dosimetry system using radiation-resistant optical fibers and a luminescent material was developed for use in a damaged Fukushima Dai-ichi nuclear power plant. The system was designed to be compact and unnecessary of an external supply of electricity to a radiation sensor head with a contaminated working environment and restricted through-holes to a measurement point in the damaged reactor. The system can detect a gamma-ray dose rate at a measurement point using a couple of optical fibers and a luminescent material with a coincidence method. This system demonstrated a linear response with respect to the gamma-ray dose rate from 0.5 mGy/h to 0.1 Gy/h and the system had a capability to measure the dose rate of more than 102 Gy/h.

Improved micro-pixel detector element for neutron measurement under high pressure

A micro-pixel detector element with robustness for high voltage was developed by an improved fabrication process, and an irradiation experiment was performed using a Cf-252 neutron source. The withstand voltage of the developed element was improved, allowing the element to operate at voltages of up to 750 V at a total pressure of 5 atm (4.1 atm of He and 0.9 atm of CF4). The measured gas gain was approximately 100, and the thermal neutron detection efficiency was estimated to be approximately 70% at a gas pressure of 5 atm. The flat-field image showed good homogeneity and an average gain spread of approximately 8.7%.