Collaboration members, from left to right: Tomoyo Nunomiya, Sunsuki Yonai, Shingo Taniguchi, Takashi Nakamura (all of Tohoku University), Sayed Rokni (RP), Michiya Sasaki (Tohoku), Stefan Roesler (CERN) and Clive Field (EB). Other members not shown: James Liu (RP), Ken Kase (RP), and Hiroshi Iwase (Tohoku). (Photo courtesy of Radiation Physics Dept.)

A team of radiation physicists recently completed a series of experiments at SLAC to determine the high-energy neutron spectra and its attenuation length in concrete. These are important quantities in the design of shielding at high-energy accelerators.

"We want to be conservative in our approach to radiation safety but need to be as accurate as possible in order to save in the cost of shielding," said Sayed Rokni, of the Radiation Physics Department (RP) at SLAC.

Rokni and Takashi Nakamura of Tohoku University have been leading a collaboration of physicists from CERN and their respective institutions in a series of experiments to reach a more accurate understanding of the energy spectrum of neutrons as they pass through shielding.

Measurements of this precision had not been possible to date, due to the lack of accurate calibrations of high-energy neutron detectors. The final calibrations of the NE213 organic liquid scintillator, which made the experiment possible, were carried out in 2001 by Tohoku University.

Nakamura’s group first used quasi-monoenergetic neutron beams to raise the calibration range to 200 MeV from the original 20 MeV. Then, at the Heavy Ion Medical Accelerator in Chiba, they used neutrons generated in the interaction of heavy ions on a thick carbon target to extend the range up to 800 MeV. A time-of-flight technique was used to determine the neutron energies.

Preliminary neutron energy spectra measured outside the FFTB dump with the NE213 scintillation counter. Measured data at different thicknesses of concrete are compared with results of simulations from FLUKA radiation generation and transport code. (Graphic courtesy of Radiation Physics Dept.)

Using these calibrations, the SLAC-CERN-Tohoku collaboration was able to conduct their radiation shielding experiment at SLAC’s Final Focus Test Beam (FFTB) in two runs in June 2001 and June 2002. Availability of beam through the Test Beam program at SLAC was crucial in the ability to perform these measurements. Ted Fieguth (EFD), Clive Field (EB), Ron Seefred (OHP) and Accelerator Department Operations staff were instrumental to the successful completion of the measurements.

The experiments were conducted by aiming the beam at the FFTB’s aluminum beam dump. The resulting bremstrahlung radiation interacts with nuclei, causing neutrons to be emitted. The neutrons and remaining gamma rays pass through iron and concrete shielding, losing energy.

At the edge of the shielding, a Bonner multi-sphere detector set measures the neutrons with energies lower than 20 MeV, and the NE213 detector measures neutrons with energies from 6 MeV to 800 MeV. Shielding width was varied as data was taken.

This is the first time that the neutron spectra have been measured in such a wide range of energy through the shielding of a high-energy electron accelerator.

Results are expected in spring of 2003, and will be used to benchmark shielding calculations in particle accelerators.

For more information, contact Sayed Rokni, Ext. 3544, rokni@slac.stanford.edu.