Angular Response

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

Currently, skin dose measurements are related to the dose at a depth of 7 mg/cm2 in tissue.

Window thicknesses of commonly available survey instruments typically range from on the order of 7

mg/cm2 to several hundred mg/cm2.

Even if the window provides only minimal attenuation, the attenuation of the beta field through the

sensitive volume of large detectors remains a problem. The detector indicates the average dose-rate

throughout the sensitive volume. The "true" dose-rate is that which occurs at the plane of the detector

incident to the radiation source. The instrument will under-respond by the ratio of this average dose-rate to

the incident dose-rate. This sensitive volume under-response is a function of the beta energy distribution

and of the size and shape of the sensitive volume.

6.3.1.2 Angular Response

The construction of most survey instruments (e.g., "cutie pie") leads to a severe angular

dependence when measuring beta radiation fields. This angular dependence results from the attenuation of

the beta field by the walls of the detector as the window is moved away from the source.

Figure 6-6 demonstrates the response of a tissue equivalent response (a 5 mg/cm2 detector under a

5 mg/cm2 window and mounted in a TE phantom) to off-axis (non- incident) 90Sr/90Y betas (energies similar

to those from uranium). Skin tissue dose response is greater to off-axis betas; survey instruments, which

effectively shield these high angle particles, will under-respond compared to skin tissue.

Figure 6-6. Measured Angular Response of the INEL TE Survey Meter to Parallel Beams of Beta

Particles From Three Standard Beta Sources

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