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A Compton camera system for use on an unmanned helicopter was previously developed to characterize the distribution of radioactive materials in highly contaminated areas. For this study, a Compton ...

In this study, we developed a 45 MeV neutron fluence rate standard of Japan. Quasi-monoenergetic neutrons with a peak energy of 45 MeV in the neutron standard field were produced by the 7Li(p,n)7Be...

An improved light-weight Compton camera exhibiting low power consumption was developed to be mountable on an unmanned helicopter to detect cesium radiation hot spots and confirm the decontamination effect of cesium-affected areas. An increase in the Ce:Gd3(Al,Ga)5O12 scintillator array from 4 × 4 to 8 × 8 and expansion of the interlayer distance enhanced the detection efficiency and angular resolution, respectively. Measurements were performed over the Ukedo riverbed in Namie, Fukushima Prefecture (Japan). The helicopter's flight path and speed were pre-programmed to lines interspaced by 5 and 10 m intervals and 1 m/s, respectively, facilitating measurements over areas of 65 × 60 m2 and 65 × 180 m2 at a height of 10 m for approximately 20 and 30 min, respectively. Results provided accurate ambient dose equivalent rate maps at a height of 1 m with an angular resolution corresponding to a position resolution of approximately 10 m from a height of 10 m. Hovering flights were executed over hot-spot ar...

Due to the accident of Fukushima Daiichi Nuclear Power Plant, some areas were contaminated by released radioisotopes (mainly 137Cs and 134Cs). Effective decontamination is demanded to encourage eva...

A quasi-monoenergetic neutron field generated in the ${^7{\rm Li}({\rm p}, {\rm n})}$ reaction consists of a high-energy monoenergetic peak and a continuum to the low-energy region. In this study, the spectral fluence of the continuum was measured with the time-of-flight (TOF) method using a ${^6{\rm Li}}$ -glass scintillation detector and an organic liquid scintillation detector for the keV and MeV region, respectively. The neutron spectral fluence was determined down to the keV region by implementing a new beam chopping system and the results showed that the neutrons that came directly from the target had a lower energy limit about 100 keV. Discussions were made also on the effect of the time-independent neutrons which are assumed to be room-scattered neutrons. The obtained information is expected to contribute to understanding the quasi-monoenergetic high-energy neutron field and improvements of calibrating neutron detectors in the field.

The unmanned aerial radiation monitoring system is very important for environmental continuous inspection and emergency inspection in severe accident. The requirement for the detector mounted in the unmanned radiation monitoring system is light weight, good energy resolution and good angular resolution. The Compton imaging is a useful method for high energy gamma ray without any physical collimator and the characteristic X-ray imaging system is also effective. Here Compton Pinhole hybrid imaging system was proposed and fabricated for the implementation in the micro UAV based radiation imaging system. The fabricated detector consists of GAGG-SiPM gamma ray detector array and has a capability of both Compton imaging and low energy X-ray imaging. The fabricated detector system is light enough for mounting on the drone.

A quasi-monoenergetic high-energy neutron field produced via 7Li(p,n) reaction consists of a high-energy peak and a continuum down to the low-energy region. The continuum is a background for the measurement for the peak neutrons. Spectral information is necessary to estimate the effect of it. Using a new beam chopping system, the spectral neutron fluence above ∼150 keV was measured by the TOF method. The lower-energy neutrons are time-independent and the unfolding method using the Bonner sphere spectrometer (BSS) is suitable to measure them. However, the common BSS with a 3He or BF3 proportional counter has difficulties in this case: signals caused by high-energy-charged particle are mixed in with the genuine signals caused by thermalized neutrons and the dead-time corrections are difficult in experiments with the pulsed-neutron beam. The gold foil is therefore adopted as a detector element for the BSS. Response matrix of the gold-activation BSS is evaluated by Monte Carlo calculations and response calibrations at a neutron standard facility of the AIST. Measurements for the quasi-monoenergetic high-energy neutron field was performed at the AVF cyclotron facility of TIARA, at the peak neutron energy of 45 MeV.

Development of three calibration fields for 45, 60 and 75 MeV neutrons has been in progress at Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) of JAEA-Takasaki. Achievement of the neutron calibration fields requires establishment of both measurement of neutron fluence and its monitoring technique. In order to measure the neutron fluence of main energy peak especially at a reference point on low fluence condition, a proton recoil counter telescope with high efficiency has been developed. The design of relatively large telescope components brought a high detection efficiency at the reference point with wide irradiation area at a long distance. Effective and precise measurement of the neutron fluence could be performed with uncertainties less than 6.5 % (standard deviation). For detecting neutrons without scattering near the target, a transmission type neutron fluence monitor has been newly developed to measure the neutron fluence directly at the collimator exit. Its performance such as counting rates was tested for various beam intensities.

Target photons mixed in the 144, 250 and 565 keV mono-energetic neutron calibration fields were measured using a cylindrical NaI(Tl) detector with 7.62 cm both in diameter and in length. The ambient dose equivalent H*(10) of the photons was evaluated by applying the “G(E) function” to the measured pulse height spectrum. Neutrons induce photons by nuclear reactions in the NaI(Tl) detector and affect the pulse height spectrum. In order to eliminate the influence of these neutron events, the time-of-flight technique was applied with operating the accelerator in the pulse mode. The ratios by the ambient dose equivalent H*(10) of the photons to the 144, 250 and 565 keV neutrons were evaluated to be 3.3%, 4.7% and 0.9%, respectively. Although high energy photons ranging from 6 to 7 MeV are emitted by the 19F(p,αγ)16O reactions, the dose of the target photons is low enough to calibrate neutron dosemeters except for ones with high sensitivity to the photons.

In Japan, several monoenergetic and quasi-monoenergetic neutron reference fields are currently operating at several research institutes over a wide energy range 8 keV–390 MeV. These neutron fields are utilized for characterizing and developing neutron detectors as well as for neutron-induced cross section studies, shielding benchmark experiments and irradiation testing of micro-electronic devices. This paper reports the current status of these monoenergetic and quasi-monoenergetic neutron reference fields.

Precise calibration of monitors and dosimeters for use with high energy neutrons necessitates reliable and accurate neutron fluences being evaluated with use of a reference point. A highly efficient Proton Recoil counter Telescope (PRT) to make absolute measurements with use of a reference point was developed to evaluate neutron fluences in quasi-monoenergetic neutron fields. The relatively large design of the PRT componentry and relatively thick, approximately 2 mm, polyethylene converter contributed to high detection efficiency at the reference point over a large irradiation area at a long distance from the target. The polyethylene converter thickness was adjusted to maintain the same carbon density per unit area as the graphite converter for easy background subtraction. The high detection efficiency and thickness adjustment resulted in efficient absolute measurements being made of the neutron fluences of sufficient statistical precision over a short period of time. The neutron detection efficiencies of the PRT were evaluated using MCNPX code at 2.61×10 −6 , 2.16×10 −6 and 1.14×10 −6 for the respective neutron peak energies of 45, 60 and 75 MeV. The neutron fluences were determined to have been evaluated at an uncertainty of within 6.5% using analysis of measured data and the detection efficiencies. The PRT was also designed so as to be capable of simultaneously obtaining TOF data. The TOF data also increased the reliability of neutron fluence measurements and provided useful information for use in interpreting the source of proton events.

The neutron calibration fields using accelerators have been developed at the Facility of Radiation Standards (FRS) of JAEA-Tokai for neutrons below 20 MeV and at Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) of JAEA-Takasaki for those over 20 MeV. At FRS, monoenergetic neutrons are generated by using proton or deuteron beam from a 4 MV Van-de-Graaff (Pelletron) accelerator. Up until the present, developments of the 8, 144, 250, 565 keV, 5.0 and 14.8 MeV fields have been completed. As procedure for the development, measurement of neutron energy spectrum, peak energy and reference fluence, development of monitoring scheme for neutron fluence and establishment of the traceability on neutron fluence to the primary standards were achieved. At TIARA, three neutron fields of 45, 60 and 75 MeV are planned to be established. Quasi-monoenergetic neutrons are generated from 7Li(p,n)7Be reaction by using proton beam from an AVF cyclotron, and led into an irradiation room through a cylindrical c...

The 8 and 27 keV monoenergetic neutron calibration fields have been developed by using (45)Sc(p, n)(45)Ti reaction. Protons from a 4-MV Pelletron accelerator are used to bombard a thin scandium target evaporated onto a platinum disc. The proton energies are finely adjusted to the resonance to generate the 8 and 27 keV neutrons by applying a high voltage to the target assemblies. The neutron energies were measured using the time-of-flight method with a lithium glass scintillation detector. The neutron fluences at a calibration point located at 50 cm from the target were evaluated using Bonner spheres. A long counter was placed at 2.2 m from the target and at 60 degrees to the direction of the proton beam in order to monitor the fluence at the calibration point. Fluence and dose equivalent rates at the calibration point are sufficient to calibrate many types of the neutron survey metres.

Evaluation of the properties for quasi-monoenergetic neutron calibration fields of high energies more than 20 MeV at TIARA is proceeding for development of the field. Among the properties needed for the development as the standard calibration field, we report on measurement of the neutron beam profile using an imaging plate with a polyethylene converter and on estimation of the contribution of scattered neutrons into the irradiation field based on pulse height distribution at various off-beam positions measured using an organic liquid scintillation detector.

The spectral neutron fluence was measured by a combination of the TOF method (above keV region) and the Bonner unfolding method (whole energy region), for a 45-MeV quasi-monoenergetic high-energy neutron field. The extrapolated TOF-based spectrum information was used as a default spectrum for the Bonner unfolding method. The Bonner measurements were performed using the 3He-counter based Bonner sphere spectrometer (BSS) and the gold-activation BSS. While both unfolding reproduced reasonable solutions, difference in amount of the low-energy neutrons was observed between the types of BSS.

A quasi-monoenergetic neutron field generated in the 7Li(p,n) reaction consists of a high-energy monoenergetic peak and a continuum to the low-energy region. The continuum is a background for the response measurements of neutron detectors for the high-energy-peak neutrons and the effect of the continuum should be properly considered. A spectral fluence of the continuum is typically measured by time-of-flight (TOF) methods with an organic liquid scintillation detector. The lowest energy of this method is limited to several MeV because of a high repetition rate of proton beams generated by the AVF cyclotron. Recently, a new beam chopping system available at the TIARA AVF cyclotron facility achieved 1/80 reduction of the beam repetition rate and the lower limit energy of the TOF method has decreased to 20 keV. A Li-glass scintillation detector was also used for measurements of keV region, which is below the working range of the organic liquid scintillation detector. The spectral fluence of the quasi-monoenergetic high-energy neutron field measured by the TOF method using the two scintillation detectors is reported in this presentation.