Fast chopper for single radio-frequency quadrupole bunch selection for neutron time-of-flight capabilities

Abstract

A fast chopper system has been developed and tested for single bunch selection with the radio-frequency quadrupole (RFQ) accelerating element of the Soreq Applied Research Accelerator Facility (SARAF). The fast chopper consists of a high voltage (HV) deflector just before the RFQ, providing both positive and negative HV deflections and fast HV switching between polarities to enable momentary transmission of a single prebunch to the RFQ. Presently, the system enables single bunch selection for protons and deuterons at a repetition rate as determined by the user of up to 200 kHz, with bunch transmission of up to 50%, and with neighboring bunch contamination of less than 15%. Single bunch selection provides SARAF with fast neutron time-of-flight (TOF) capabilities. Measurements performed with liquid scintillation detectors show clear gamma and neutrons peaks, with TOF resolution of about 1 nanosecond FWHM. Beam dynamics simulations suggest possibilities for further improvements of the fast chopper and single bunch selection characteristics, with a significant lowering or elimination of the neighboring bunches, enhanced TOF resolution, and increased repetition rate to above 200 kHz. Fast neutron TOF capabilities, especially at phase II of SARAF, will provide exceptional opportunities for neutron induced reaction measurements for nuclear technology and fundamental research.

Upgrade of SARAF fast beam chopper and its applications in Phase I and Phase II of SARAF linac

“…The single-bunch repetition rate was obtained by scaling down the SARAF RF frequency of 176 MHz, provided by the main oscillator to the desired usable chopper repetition rate. This scaling down of the frequency and control of the trigger (delay, duty cycle) was done by using standard nuclear instrumentation modules [7]. The system downscaled the master 176 MHz RF signal while maintaining the original RF phase.…”

“…The first of these tests was a study of the 56 Fe(𝑛, 𝛾𝑛 ) reaction. Monochromatic neutrons were produced by 3 MeV protons interacting with a thin primary LiF target via a 7 Li( 𝑝, 𝑛) 7 Be reaction. The target was placed in a chamber, which was situated after the PSM module.…”

“…For a detailed explanation and test results of the fast chopper prototype system, please see ref. [7].…”

Design study of a single bunch selection method for neutron TOF experiment at RAON heavy ion accelerator

“…However, such a repetition time is too short for high-resolution neutron time-of-flight (TOF) experiments. To address this issue, several single bunch selection methods have been proposed [1][2][3][4][5][6][7]. The Soreq Applied Research Accelerator Facility (SARAF) proposed and tested a scheme based on a fast chopper [1,2].…”

“…To address this issue, several single bunch selection methods have been proposed [1][2][3][4][5][6][7]. The Soreq Applied Research Accelerator Facility (SARAF) proposed and tested a scheme based on a fast chopper [1,2]. Therein, a CW beam is chopped into a pulsed beam in the low-energy beam transport (LEBT) region by varying the electric potential of the chopper electrode in a very short time (∼ 28 ns).…”