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DOE-STD-1128-98
6.3.6.2 Tissue Equivalent Proportional Counter
The tissue equivalent proportional counter (TEPC) is not often used by
health physicists, but it can provide highly accurate estimates of dose
equivalent. The TEPC consists of a hollow sphere or cylinder of tissue
equivalent plastic filled with low-pressure equivalent gas. The pressure is
so low (a few torr) that the TE gas cavity has the same mass stopping
power as a 2-Ám sphere of tissue at unit density. Because the TEPC
actually measures the energy absorption in a known mass of tissue
equivalent material, it provides an absolute measure of absorbed neutron
dose. The TEPC also measures the pattern of microscopic energy
distributions from any penetrating ionizing radiation. With appropriate
algorithms, LET distributions, hence quality factors, can be calculated.
Thus, the TEPC provides absorbed dose, quality factor, and dose
equivalent from a single spectral measurement of the event size
distribution from the TEPC.
The TEPC can provide highly accurate measurements of dose equivalent
under laboratory conditions. The TEPC can measure dose equivalent
within ▒5% to ▒10% when exposed to NIST-calibrated 252Cf sources
(Brackenbush et al., 1991). However, it suffers from stability problems,
and its accuracy decreases with time as impurities diffuse from the TE
plastic walls and temperature changes cause gain shifts in the
proportional counter. Nevertheless, the TEPC can provide reasonably
accurate measurements of dose equivalent in the workplace (▒15%) over
extended time periods of 6 months or more, and can be used to monitor
dosimeter irradiations on phantoms in the workplace.
6.3.6.3 Liquid Scintillator Spectrometer
The liquid scintillator spectrometer typically consists of a 2-in. by 2-
in.cylindrical cell of hydrogenous scintillator solution in contact with a
photomultiplier. Neutrons interact in the scintillator to produce proton
recoils, which interact with the scintillator to produce light. With careful
calibration, the incident neutron energy spectrum can be unfolded from
the measured distribution of scintillation events.
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