Time-of-flight system with a movable ion detector for absolute measurement of cyclotron beam energy

サイクロトロンビームエネルギー絶対値計測のための検出器可動型飛行時間測定システム

奥村 進; 倉島 俊; 宮脇 信正; 吉田 健一; 福田 光宏

Okumura, Susumu; Kurashima, Satoshi; Miyawaki, Nobumasa; Yoshida, Kenichi; Fukuda, Mitsuhiro

サイクロトロンのビームエネルギー絶対値を計測するために、飛行時間測定法による計測システムを開発した。検出器にはマイクロチャンネルプレート検出器とプラスチックシンチレーション検出器を用い、高時間分解能での飛行時間計測を達成した。短い飛行距離で高精度のエネルギー計測を可能にするために、プラスチックシンチレーション検出器を移動し、異なる飛行距離に対する飛行時間差を用いることで、大幅に誤差を抑制した。その結果、サイクロトロンで加速する高エネルギービームエネルギー計測を0.1%台の精度で可能にした。また、サイクロトロンビームが加速RFに同期したパルスビームであることを利用して、本計測システムによってプラスチックシンチレーション検出器のみによるエネルギー計測も実現した。

A time-of-flight system for determining the absolute energy of ion beams has been developed for the azimuthally-varying-field (AVF) cyclotron at Japan Atomic Energy Agency (JAEA). Ion detectors, a microchannel-plate detector and a plastic scintillation detector, were applied to the system in order to achieve high time resolution measurement of the flight time and to cover a wide range of beam intensity in combination with a beam attenuator. The change of the flight length, performed by moving the plastic scintillation detector with flexible bellows, allows determining the mean beam energy only from the relative measurement of the flight time and the flight length without knowing their absolute values. The maximum movable distance of 2 m yields the difference in the time-of-flight from 16 to 94 ns for the energy range of ion beams accelerated by the JAEA AVF cyclotron. The time-of-flight system even with the 2 m change in the flight length achieves accurate energy determination, since the relative measurement has an advantage of elimination of the uncertainties in determination of the time-zero and the length-zero. The time-of-flight measurement with the relative measurement also allows estimation of the beam energy spread without conventional expensive systems such as a magnetic spectrometer. By comparing the widths of the time-of-flight spectra of ions accumulated at different flight lengths, we have estimated the energy spread. In order to reduce a labor process of the coincidence detection of ions in the time-of-flight measurement, we have also accomplished a simple estimation method for mean beam energy determination with a beam bunch, naturally modulated with the acceleration RF of the cyclotron. The mean beam energy has been obtained from the shift of the beam bunch centroids at different flight lengths in the time-of-flight spectrum.