Source-Detector Geometry

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

6.3.1.3 Source-Detector Geometry

Measurements taken close to small beta sources may be inaccurate due to non-uniform

irradiation of the sensitive volume of the detector. Uranium in most DOE facilities tends to present

wide-area sources of beta radiation. However, adjustments would need to be made if significant non-

uniform irradiation was encountered.

6.3.1.4 Detector Construction and Use

Characteristics of instrument construction may significantly affect their response and use. For

example, many survey instruments have "beta windows" that are intended to discriminate between beta

and gamma radiation. Obviously, measurements of beta dose-rate must be made with the beta window

open. It should be noted, however, that a number of instruments have beta windows that are only a few

hundred mg/cm2 thick. Such windows can transmit a significant fraction of the dose-rate from high-energy

beta-emitters (e.g., 234mPa). Thus, up to 10% or 20% of the "gamma only" reading may be due to the

higher-energy betas penetrating the so-called beta window.

Occasionally, survey instruments are placed in plastic bags or covered to protect them from

becoming contaminated. Bagging the instrument places additional absorber between the radiation field and

sensitive volume of the detector. Calibration of the instrument (or application of a correction factor) should

take this additional shielding into account.

6.3.2 Portable Survey Instruments --Gamma Radiation Response

Although the external dose resulting from gamma and x-ray radiation from bare uranium is a small

fraction of the total, it represents the "penetrating" or whole body dose source and is the only source of

radiation from contained facilities (i.e., those having glove boxes, etc.). Survey instruments are typically

calibrated with 137Cs (0.663-MeV) photons. Typical portable survey instruments demonstrate a fairly flat

energy response above 250 keV, while the response below 250 keV can be variable to a greater or lesser

degree depending upon the instrument design. Figures 6-7 and 6-8 show average response of a group of

commercial survey instruments. Figure 6-9 shows a typical gamma spectrum from a uranium oxide source

while Table 6-8 illustrates the wide variation that can occur in the photon spectra at various locations in a

single plant. This demonstrates the desirability of using ion chambers or compensated beta instruments for

dose-rate measurements. It also indicates the need to have knowledge of the energy response of the

instrument used and the value, or at least qualitative knowledge, of the photon spectra at the various work

stations.

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