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quantum states have been presented in Refs. [1, 2, 3]. We have proposed a more precise measurement using a pixel detector with an image magnification system [4]. Another pixel detectors for UCN have been developed recently, such as that reported in Ref. [5]. In this article we present the development of a pixel detector based on a commercial charge coupled device (CCD) covered with a neutron converter. By comparing $^{10}$B and $^{6}$Li as converter materials, we find that a $^{6}$Li converter produces energetic tritons which penetrate deep into the CCD in various directions, degrading the spatial resolution. Hence we conclude that $^{10}$B is an appropriate material for a neutron converter. 

## 2. Detector design 

The developed detector consists essentially of a CCD covered by a neutron converter. Charged particles produced via nuclear reaction in the converter are detected with the CCD. The choices of converter material and CCD are key for this detector. 

## 2.1. Neutron converter 

$^{10}$B and $^{6}$Li are chosen as test materials for the neutron converter, because of their large cross-sections with neutrons. The neutron absorption cross- sections for $^{10}$B and $^{6}$Li are 4 . $^{01}$× 10 3 and 0 . $^{95}$× 10 3 barn, respectively, for thermal neutrons ( v = 2,224 m/s). 

Neutrons react with $^{10}$B and $^{6}$Li in the following processes: 

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<img src='content_image/336725.jpg'>

