Development of Real-Time Measurement of Effective Dose for High Dose Rate Neutron Fields Page: 2 of 6
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Executive summary: Studies of the biological effects of neutrons, and other
applications of neutron irradiation facilities, require both a source of neutrons and a way
of characterizing the radiation exposure. In the case of neutrons and other high LET
radiations, the dose (energy deposited per unit mass) is not sufficient to describe the
exposure, since most effects of this radiation depend on the "quality" of the radiation as
well as the energy deposited. No ideal way of specifying radiation quality has yet been
found, but neutron energy, the linear energy transfer of the secondary radiation, and the
lineal energy have been used in formulating different definitions of quality. Depending
on the source of the radiation it may be relatively easy to calculate the neutron energy
spectrum, or it may be very difficult. It is almost always difficult to measure the neutron
spectrum. If the Neutron spectrum can be evaluated, then the LET distribution can be
calculated, but it can not be measured directly. The easiest of the three quantities to
measure is the lineal energy. At low dose rates, where individual energy deposition
events (passage of a secondary particle through a small volume) can be measured, the
probability density of lineal energy can be measured directly and average values such as
the dose mean lineal energy can be calculated. However, most experiments require dose
rates that are two high to measure as individual energy deposition events in detectors of
The variance method provides an alternative way of measuring the dose mean lineal
energy, without having to measure individual events. This method is based of measuring
the total dose deposited in a specified time interval. This dose is the sum of the energy
deposited by each interaction occurring during that time, and because the number of
events is relatively small, the energy measured in identical time intervals will vary. The
variance of the dose will depend on the size and number of individual events producing
the dose, the larger the individual events, the greater the variance. It can be shown that
the dose mean lineal energy can be calculated from the measured variance of dose in
identical samples if the source dose rate is constant.
The electronics and detectors necessary to measure the dose and dose mean lineal energy
in mixed neutron and gamma ray radiation fields with dose rates in excess of 1
Gy/minute have been developed. This is the dose rate that would typically be used for
radiation biology experiments using filtered radiation from a research reactor. To
achieve this capability, a specialized current integrator which produces output pulses
proportional to the dose received in a specified interval (typically 100 ms) and small
spherical ion chambers, 4.3 mm in diameter, were developed. The output of the current
integrator was processed by a multichannel analyzer and software to calculate the mean
and variance of the charge. The ion chamber is operated at or below atmospheric
pressure to simulate tissue volumes ranging from a few nanometers to a few micrometers
This system has been used to measure dose and dose mean lineal energy for radioactive
sources and for reactor neutron fields at high dose rate. The results are consistent with
calculated from single event data measured at much lower dose rates in radiation fields
which approximate the spectra of the high dose rate fields needed for biology
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Braby, L. A.; Reece, W. D. & Hsu, W. H. Development of Real-Time Measurement of Effective Dose for High Dose Rate Neutron Fields, report, August 29, 2003; United States. (digital.library.unt.edu/ark:/67531/metadc736039/m1/2/: accessed September 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.