[Paper Review] Pulsed Neutron Source using 100-MeV electron Linac at Pohang Accelerator Laboratory

G. N. Kim, Heung-Sik Kang+4 more|ArXiv.org|Aug 15, 2000

Nuclear Physics and Applications3 citations

TL;DR

This paper presents the design, construction, and initial testing of a 100-MeV electron linac at Pohang Accelerator Laboratory to produce a pulsed neutron source via bremsstrahlung and photo-nuclear reactions in a tantalum target. The facility achieved a 15-meter time-of-flight path with a measured length of 10.81±0.02 m, enabling neutron energy resolution for nuclear data measurements using TOF spectroscopy with 0.5 µs channel resolution.

ABSTRACT

The Pohang Accelerator Laboratory operates an electron linac for the pulsed neutron source as one of the long-term nuclear R&D programs at the Korea Atomic Energy Research Institute. The designed beam parameters are as follows; The nominal beam energy is 100 MeV, the maximum beam power is 10 kW, and the beam current is varied from 300 mA to 5A depends on the pulse repetition. The linac has two operating modes: one for short pulse modes with the various repetitions between 2 ns and 100 ns and the other for long pulse modes with 1 ms repetition. We have constructed and tested an electron linac based on the existing equipment such as a SLAC-5045 klystron, two constant gradient accelerating sections, and a thermionic RF-gun. We present the characteristics of the linac and report the status of the pulsed neutron source facilities include a target system and time-of-flight paths.

Motivation & Objective

• To develop a pulsed neutron source for high-resolution nuclear data measurements using electron-driven photoneutron production.
• To utilize existing infrastructure at Pohang Accelerator Laboratory to construct a cost-effective, high-intensity neutron facility.
• To validate the time-of-flight path length and neutron energy resolution for future nuclear cross-section measurements.
• To demonstrate beam performance and neutron yield from a 100-MeV electron linac with a Ta target and water cooling system.
• To establish a reliable data acquisition system for neutron TOF spectroscopy with 16,384-channel time resolution.

Proposed method

• Constructed a 100-MeV electron linac using existing components: SLAC-5045 klystron, two constant gradient accelerating structures, thermionic RF-gun, and beam diagnostics.
• Employed a 15-meter stainless steel TOF tube perpendicular to the linac, with 6Li glass scintillator and BF3 counter for neutron detection and beam monitoring.
• Used a 150 MHz time digitizer (Turbo MCS) with 0.5 µs channel width and 12 Hz RF trigger for time-of-flight data acquisition.
• Applied least squares fitting to resonance energy and channel number data to determine flight path length using the equation: L = (3.72 × ∆t × I + τ) / ∆E.
• Conducted neutron TOF measurements using Sm, Ta, W, and Ag samples with a 0.5 mm Cd filter to suppress thermal neutrons.
• Shielded detectors with polyethylene and lead to reduce gamma flash and thermal neutron background.

Experimental results

Research questions

• RQ1 What is the effective flight path length of the 15-meter TOF neutron path at Pohang Neutron Facility?
• RQ2 What beam parameters (energy, current, pulse width) are achievable with the 100-MeV electron linac using existing components?
• RQ3 What is the neutron yield per kilowatt of beam power from a 100-MeV electron beam on a tantalum target?
• RQ4 How well does the time-of-flight system resolve neutron energies with 0.5 µs channel width?
• RQ5 What is the measured energy spread and emittance of the electron beam at 75 MeV?

Key findings

• The measured flight path length of the TOF system is 10.81 ± 0.02 meters, determined via least squares fitting of resonance energy and channel number data.
• The electron linac achieved a maximum beam energy of 75 MeV with a beam current of 40 mA at the end of the linac, and a 1.8 µs pulse width at 12 Hz repetition rate.
• The energy spread was reduced to less than 1% through RF phase and alpha magnet field optimization.
• The neutron yield from the Ta target was estimated at 2.0×10^12 neutrons per second per kilowatt of beam power.
• The time-of-flight system achieved 0.5 µs channel resolution using a 16,384-channel Turbo MCS digitizer.
• The delay time τ between the RF trigger and actual zero time was measured as 0.87 µs, critical for accurate energy calibration.