The necessity of neutron dosimetry according to the development of the radiotherapy using accelerator keeps increasing on not only the nuclear installation and the high energy physics experiment facilities but also the medical treatment sites. Here, it explains the development and current state of dosimeters used for such measurements.

  Many neutron measurement devices such as personal dosimeters and neutron survey meters are used in neutron radiation fields. The neutron measurement devices have given a reliable value for the fast neutron fluence or the dose equivalent by periodical calibration. This paper reports the reference neutron field used in the calibration, calibration method of the devices and determination of the neutron fluence in the neutron standard field of the National Institute of Industrial Science and Technology.

  In Japan, the National Metrology Institute of Japan has been providing the national standard on neutrons, which plays an important role in securing the reliability of sensitivities of neutron detectors and intensities of neutron sources. This paper reports the present status and new developments in neutron standardization. Worldwide consistency of the neutron standard has been supported by participating in the key comparisons organized by the International Committee of Weights and Measures. Since October 2006, calibrations based on our neutron standard have been accepted in worldwide within the range specified in the calibration and measurement capabilities in the framework of the Mutual Recognition Arrangement. We are renovating the traceability of neutron measurement in Japan by starting calibration services based on the Japan Calibration Service System.

  Clinical trials for boron neutron capture therapy (BNCT) are performed using research reactors in world-wide. BNCT is a radiation therapy for refractory cancer as malignant brain tumor. To perform BNCT procedure based on proper treatment planning, JCDS, a treatment planning system for BNCT has been developed by Japan Atomic Energy Agency (JAEA). Practical application of JCDS enabled to perform the BNCT with the epithermal neutron beam in JRR-4.

  JCDS is being improved to apply accelerator based BNCT. One of features of new JCDS is that PHITS has been applied to particle transport calculation. PHITS is a multi-purpose particle Monte-Carlo transport code. Hence application of PHITS enables us to evaluate doses not only for neutrons and photons but also for protons and heavy ions. To verify calculation accuracy of PHITS for BNCT dosimetry, dose evaluations for neutron irradiation of a cylindrical water phantom and for an actual clinical trial performed at JRR-4 were performed, then the results were compared with calculations obtained from conventional JCDS. Calculations of neutron and photon fluxes and of several doses were in good agreement with conventional JCDS calculations, these results demonstrated that the new JCDS with PHITS is applicable to BNCT treatment planning in practical use.

In charged particle radiotherapy, a part of primary particles collides with nuclei and is broken into various particles through a fragmentation reaction. Among them, neutrons are most widely spread out due to their neutral charge and cause unwanted dose exposure to far away from the target region, including healthy tissue of the patient. In this study we have attempted to measure the dose delivered by neutrons to surrounding normal tissues in charged particle radiotherapy from the viewpoint of estimating the risk of their causing secondary cancer to the patient. A thick water phantom was irradiated with beams of ¹²C-290 MeV/n or ¹H-160 MeV. Bonner sphere (⁶Lil(Eu) scintillator inserted) was placed around the phantom in order to measure the emitted neutrons. A commercial neutron rem counter was also used in the measurement for the sake of comparison. The measurements suggest that the dose delivered by neutrons is less than 1 % of the total dose delivered by carbon ions. This dose level was found to be less than that obtained with protons.

  Recently the traceability system (JCSS) of neutron standard based on the Japanese law “Measurement Act” has been instituted. In addition, importance of the neutron dose evaluation has been increasing in not only the neutron capture medical treatment but also the proton or heavy particle therapy. Against such a background, a symposium “Neutron dosimetry in neutron fields - From detection techniques to medical applications-“ was held on March 29, 2008 and recent topics on the measuring instruments and their calibration, the traceability system, the simulation technique and the medical applications were introduced. This article summarizes the key points in the discussion at the symposium.