Development of Real-Time Measurement of Effective Dose for High Dose Rate Neutron Fields Page: 4 of 6
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Fig. 1. The icrodosimetric detector consists of a mini spherical
tissue-equivalent ion chamber, an Alurinum container and a
Charge integrator: For the dose rates likely to be encountered in radiation biology
exposure facilities, and for the detector sizes appropriate for use in most exposure
facilities, variance measurements can best be made with a charge integration time on the
order of 0.1 seconds. Several charge integrating circuits, suitable for integration times in
this range, were evaluated. The result was that the most satisfactory was also very simple
and inexpensive. It is based on integrating the charge on the capacitance of a FET, and
type NTE326 was found to work well for the purpose. An integrated circuit timer,
SPG8651A, was used to generate gate pulses at 100 ms intervals, and an operational
amplifier, LT1793, is used to amplify the voltage pulse that is produced across a 1 x 106
ohm resister from the FET source to ground. This simple system proved to be useful for
integration times ranging from 10 to 500 ms, with very good linearity and leakage current
noise of 2.3% at 1.28 x ao-13 amps. Because of the digital nature of the reading process,
the average leakage current is easily subtracted from the result.
Test chamber: For initial tests of the charge integrator and variance method, an existing
small ion chamber was used. This chamber was a 6 mm long, 3 mm diameter cylinder
with hemispherical end and 0.9 mm diameter anode. Although the complex path length
distribution in this volume, and the lack of provisions for operation below atmospheric
pressure limited the use of this detector, it provided the opportunity to test the rest of the
system while the spherical, low pressure detector described above was being assembled.
Results: Using the cylindrical test chamber, which simulated an annular site with a
mean chord length of 3 micrometers, variance measurements were made in a 137Cs
irradiator, in x-ray fields at 100, 140, 200, and 240 kV, and in mixed neutron and gamma
fields with and without an added 4 cm water moderator. In these different fields, the
measured dose rates varied from 0.08 to 0.9 Gy/minute, and were consistent with other
measurements of the dose rate at the same locations. Measured values of the dose mean
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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/m1/4/: accessed November 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.