I. Introduction
The neutron imaging plate (NIP) made of storage phosphors and neutron sensitive materials has made a great success in the field of neutron scattering studies [1]. However, the NIP is sensitive not only to neutrons but also gamma rays. To overcome this problem, new NIPs consisting of BaFBr: Eu and Dy2 O3 have been developed [2], where the neutron image was obtained by activation of Dy after the direct neutron image was erased with intense lightning. Another way to solve this problem is to use photostimulable phosphors that consist of low Z materials such as KCl: Eu or KBr: Eu and LiF as a neutron sensitive material [3]. Low-Z storage phosphors that contain neutron sensitive materials in the base matrix would be alternatives for the purpose, where Z denotes an effective atomic number of the phosphor. Recently, new storage phosphors for thermal neutron detection have been reported [4] [5] [6], in which the phosphors contained atomic boron in the base matrix. On the other hand, the authors have studied characteristics of SrBPO5: Eu [7], [8], CaBPO5: Ce [9], and SrBPO5: Ce [10] phosphors for the purpose. The phosphors also have a low density compared to that of BaFBr: Eu , which is usually used as a phosphor material of NIP. It will be favorable for reducing gamma-ray influence on the signal in the field where both neutrons and gamma rays exist because phosphors with lower densities are less sensitive to gamma rays. Although the photostimulated luminescence (PSL) yields of these phosphors were small compared to those of commercially available NIPs, the authors have reported that the PSL yields of CaBPO5: Ce and CaBPO5: Ce phosphors were increased by introducing fluorine atoms [4], [5]. In this paper, characteristics of RBPO5 (R=Ca, Sr)-based storage phosphors for neutron detection are described, including the results of optimization of amounts of fluorine atoms and estimation of the detective quantum efficiencies (DQEs).