Scan Survey for Fixed Contamination - STD-1128-98master0112

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

If contamination is detected during a scan survey for fixed contamination, a survey

for removable contamination should be performed to determine if the

contamination is fixed and to quantify any removable contamination. The survey

should be performed using a small piece of absorbent material, such as a standard

paper smear. This type of survey for removable contamination is often called a

technical smear survey. If no contamination above the guideline values for

removable contamination in Table 4.1 is detected during the smear survey, the

contamination is fixed, and the area should be posted appropriately.

A technical smear survey may be used routinely to detect removable

contamination, especially for contamination surveys of radiological areas.

Scan Survey for Fixed Contamination

A scan survey for fixed contamination requires passing a portable instrument over

the surface of the area being surveyed at a fixed, known scan speed and at a

specified distance from the surface. Typically, the scan speed is 2 in./s (5 cm/s)

and the maximum distance is 0.25 in. (0.6 cm) for alpha contamination instruments,

but this can vary depending on the instrument, probe configuration and

background. A scan survey should be used to survey material that resides in an

area controlled for contamination purposes, an area where unsealed radioactive

sources are used, or a radiological buffer area surrounding an area controlled for

contamination purposes. A scan survey in conjunction with a wipe survey should

be used to release from radiological control material with a total surface area less

than 5 ft2 (0.46 m2). A statistically based survey, which will be discussed later,

should be used to release from radiological control material with a surface area

greater than 5 ft2 (0.46 m2).

During the performance of scan surveys, the audible response of the instrument is

faster than the needle deflection. Therefore, audible response should be used in

conjunction with meter readings. For alpha surveys, the surveyor should pause for

3 to 5 seconds each time an individual pulse is detected in order to allow a longer

count time at the location of the detected pulse, until it is determined whether the

response indicates random background noise or detected contamination.

Several important factors affecting scan survey detection sensitivity are: instrument

detection efficiency, background, size of the effective probe area, and the speed at

which scan surveys are performed. For a given instrument, scan speed can be a

critical factor as counting time is inversely proportional to scan speed. For

instruments with larger detector faces, the scan speed is faster for a given rate of

meter movement because a point on the surveyed surface remains beneath the

4-16