LBL Scientist develops remmeter

June 4, 1993

By Mike Woolridge, [email protected]

In many ways, the neutron is a rather unassuming particle. Lacking the charge of its atomic partners the proton and electron, the free neutron interacts only slightly with other matter. Yet when it happens to hit something head on, a molecule in your body for instance, the result can be far from mild.

You may not feel anything in a strong neutron field, explains Rai-Ko Sun, a physicist in the Environmental Health and Safety Division, "but over time it can destroy cells and tissue."

Sun is developing the first device to accurately measure a person's exposure to high energy neutrons. His device, called a remmeter, should find immediate application at powerful accelerators such as the Super Conducting Supercollider, where particle collisions within can jar neutrons loose from atomic nuclei. Although such accelerators are walled with radiation-blocking concrete, there hasn't been an easy way to tell how well the slabs kept higher energy neutrons inside.

When fully operational, Sun's device should be able to measure neutrons at energies greater than 1 GeV, energies 100 times higher than the limits of current devices.

Sun's device is an improved version of an Andersson-Braun remmeter, the bread-box-sized apparatus that has been the standard neutron measuring device for three decades. Remmeters measure the levels of radiation absorbed by a person's body.

The Andersson-Braun model detects neutrons with a container of boron trifluoride (BF3) gas surrounded by several layers of poleyethylene plastic.

The polyethylene serves to slow incoming neutron particles. By the time a speeding neutron has rattled through the layers of plastic, it is moving slowly enough to be measured by the gas inside.

Neutrons collide with the boron atoms of the BF3 gas, generating smaller, charged particles. By reading the resulting electrical charge inside the container, the remmeter approximates how much neutron radiation a person is being exposed to.

But very fast neutrons go through the device without hitting anything, says Sun. The device looks transparent to them. Consequently, the Andersson-Braun remmeter is of little use in detecting the potentially more dangerous neutrons above 10 MeV.

Building on theoretical work published by Italian physicists in 1990, Sun constructed a remmeter that appears to be able to handle the faster particles.

The key to Sun's design is a layer of lead sandwiched between the layers of plastic. Unlike polyethylene, lead can rein in the high energy neutrons so they register with the boron trifluoride gas. And since lead doesn't affect lower energy neutrons, its resonance properties allow them to zip through the extra layer of metal wont compromise the devices sensitivity at lower energies.

Sun first tested his new remmeter with mathematical simulations. These complex Monte Carlo calculations took into account the device's proposed composition and shape, then estimated its performance in neutron fields at various energies.

Sun then built a remmeter to theoretical specifications and took it to LBL's Bevalac accelerator, where he showered it with concentrated neutron beams with energies from 24 to 2100 MeV. Although now shut down, the Bevalac was special in that it could create neutron beams at relatively precise energies, allowing Sun to easily compare his experiment with the earlier calculations.

The numbers have matched up quite well, even at the highest radiation levels. The results are very close, says Sun. The measurements are well within the acceptable error.

Sun says he hopes to have a working remmeter available to physicists within two years. In addition to accelerator facilities, he thinks the device will prove useful at environmental agencies, hospital radiation facilities, and wherever else radiation emission is a concern.