Tissue Equivalent Proportional Counter

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

spectra are distinctly different from neutron emission spectra (see Section 6.2),

which do not contain scattered or background neutrons.

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-:m 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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