RARAF – Fast Neutrons

Learn more:

• About the neutron beam
• About the neutron irradiation fixtures
• About the dosimetry for neutrons

Neutron Beam

Large numbers of 14 MeV neutrons can be produced using the T(d,n)⁴He reaction. The neutron energy, fluence, and dose rate are nearly independent of angle so that planning irradiations and designing fixtures to hold samples are relatively easy. A significant fraction of the energy deposited in tissue by 14 MeV neutrons is from alpha particles and heavy-ion recoils. Approximately 70% of the energy deposition is from proton recoils.

Currently available neutron parameters

The D(d,n)³He reaction with a Q-value of 3.3 MeV is used to produce 6 MeV neutrons by bombarding deuterium targets with 3.1 MeV deuterons. High dose rates are obtained by using thicker targets. The neutron energy varies more strongly with angle and deuteron energy than for the T(d,n) reaction because more of the available energy is coming from the incident deuterons. The reaction cross section has a strong forward peak so that irradiations are performed at angles near 0° in order to maximize dose rate and reduce the fractional dose contribution from gamma rays. At this neutron energy, about 90% of the energy deposition is from protons, the contribution from heavy recoils is less than for 14 MeV neutrons, and very little deposition is from alpha particles.

Irradiations with low-energy neutrons are performed using the T(p,n)³He reaction with a Q-value of -0.7 MeV. The neutron energy varies quite strongly with the reaction angle and the incident particle energy. For neutron energies below 0.8 MeV, irradiations are conducted at angles between 100° and 130° from the incident beam direction in order to maximize dose rate and minimize energy spread. Low-energy neutrons with energy near 440 keV are biologically most effective because almost all of the energy is deposited by protons near the Bragg peak which have high LETs.

A new facility for the production of useful dose rates of neutrons with dose-mean neutron energies as low as 25 keV (slow neutrons) has been developed using the ⁷Li(p,n)⁷Be reaction. A water-cooled rotating target is employed to prevent evaporation of the lithium by beam currents of 100 µA.

Neutron Irradiation Fixtures

Neutron irradiations are performed in the SH and SV caves. Normally, a horizontal charged-particle beam is used but a vertical beam is available for irradiating biological systems for which it may be better suited. Neutron irradiation fixtures for radiobiology and physics are designed in consultation with the experimenter to meet the needs of both the researcher and the dosimetrist.

Although the target assemblies were designed to minimize absorbing material, the neutron dose at large angles from the beam direction is not azimuthally uniform. To irradiate large numbers of samples uniformly, most fixtures provide a means of rotation about the beam axis. We have a Ferris wheel-like fixture used to irradiate rats and mice, flasks, test tube and dishes.

Above, the wheel has been modified for irradiation of cell monolayers growing in commercial cell culture flasks. This arrangement has been used in studies of transformation induction in mouse cells.

Higher dose rates to cells are delivered using a fixture mounted on the vertical beam line. This apparatus was originally used for irradiating hamster cells in small vials made from plastic pipettes.

Dosimetry

Dosimetry for neutron irradiations is performed using tissue-equivalent (TE) ionization chambers for total dose measurements and neutron-insensitive dosimeters to measure gamma-ray dose. The dosimetry measurements are relative to the response of a TE ionization chamber in a fixed location which is used as a monitor. Radiation doses are then delivered based on the response of the fixed monitor chamber. The gamma-ray dose and the incident beam current on the target are also monitored.

The total dose ionization chambers have walls made of A-150 muscle TE plastic and have methane-based TE gas sealed inside or flowing through. Insulators are made of tissue-like materials such as styrene or Lucite. The chamber is placed at the same position as the sample would be during the irradiation. A chamber is selected so that the chamber volume is similar to the sample volume, and the wall thickness is adjusted to match the amount of material between the center of the sample and the neutron-producing target. Chambers of various sizes and geometries from 1/4" diameter spherical to 1" diameter by 3/16" deep parallel plate arrangements are available so that measurements can be made even for small volumes or large areas.

If the sample is rotated about the beam axis or if there are several samples being irradiated simultaneously, measurements are made at various positions around the target.

Gamma-ray dosimetry is performed using either a compensated Geiger-Mueller dosimeter or an aluminum-walled, argon-filled (Al-Ar) ionization chamber. The large response of the Al-Ar chamber to 14 MeV neutrons makes it unfeasible to use for that energy. Measurements of gamma-ray dose often cannot be made at the sample position with the G-M type dosimeter but are made as close to it as possible. The gamma-ray dose rate is essentially isotropic about the target so that only inverse square law corrections are necessary.

The dosimeters are calibrated using either a 50 mg radium-226 or 7 Ci cesium-137 gamma-ray source, both of which have been calibrated by the National Bureau of Standards.

Corrections to the dosimetry measurements are made for any positional differences, for dose buildup or attenuation factors due to differences in material thickness, for variations of W_n with neutron energy, and for neutron kerma difference due to composition differences between the samples and the chamber. Computer calculations of the mean neutron energy, energy spread, and the relative dose rate at various points on the sample are provided.

The electrometers used to measure the currents (typically of the order of 10^-9-10^-13 A) from the ionization chambers were designed by R. Mills and fabricated at RARAF. The higher-impedance circuitry has been potted to minimize parasitic volume (shows almost no response to radiation) and is attached directly to a chamber to eliminate sensitivity to movement. The low-impedance control circuitry is located in a NIM module at the console.

Site developed by CE, page last modified by JL on November 29, 2007 .

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