Instrumentation Considerations cont'd

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiological Protection in Uranium Facilities

The instrument should have a minimum battery lifetime of 200 hours of continuous

operation. ANSI N42.17A specifications differ slightly.

The response of the instrument should not change by more than 15% from a reference

value taken at 20C over the anticipated temperature range for operation.

The instrument system should function within specifications over all anticipated

combinations of temperature and humidity (e.g., 15 to 65C, 40% to 95% relative

humidity).

ANSI N317 states the minimum detection capability for alpha monitoring instruments ideally

should be 220 dpm/100 cm2 of surface area and should not be more than 500 dpm/100 cm2. This

requirement should be met in the presence of a radiation field of 0.10 rem/h of neutrons in the energy range

of thermal to 10 MeV, and/or in the presence of 0.10 rem/h of photons in the energy range of 0.010 to 1.25

MeV. The operating range should be from 0 dpm to at least 100,000 dpm/100 cm2 of surface area. The

response of the instrument to beta-interfering radiation is an important specification that should be stated

by the manufacturer.

Photon monitoring instruments should meet the accuracy requirements stated in ANSI N317 over

the energy range of 0.01 to 1.25 MeV. The angular response of this type of instrument should be within

15% over a 2 pi steradian frontal direction using at least two photon sources with energies ranging from

0.06 to 1.25 MeV. Experience has shown this response specification is not met by most instruments at

lower energies due to attenuation of the photon. The energy dependence should be within 15% over the

range of 0.01 to 1.25 MeV and the operating range should be from 0.5 mR/h to at least 5000 mR/h.

Experience has shown that 20% over 0.01 to 1.25 MeV is more realistic. This specification applies to a

specific window selection (e.g., below 0.05 MeV, the electron equilibrium cap or beta shield must be

removed).

ANSI N42.17A has a broader scope than ANSI N317, but the criteria in it apply to portable

survey instruments. Additional criteria include geotropism (maximum change of 6% from reference

reading for all orientations), temperature shock, mechanical shock, vibration, and ambient pressure

(maximum change of 15% from reference reading for the latter four criteria). Some differences exist

between ANSI N42.17A and ANSI N317. In most cases, the criteria for ANSI N42.17A are more

applicable because these criteria are based on substantial testing, which was sponsored by DOE. In ANSI

N42.17A, precision is tied into a measurement level; for example, it quotes a precision of 15% at <500

cpm and 10% at >500 cpm. Also, with the advent of liquid crystal displays and other digital readouts,

"response time" is defined as the time it takes for the reading to move from 10% to 90% of the equilibrium

or steady-state reading. Another significant difference in the standard is the battery lifetime specification

is 100 hours instead of the 200 hours mentioned in ANSI N317.

For direct alpha contamination surveys, the use of audible signals (headphones or speaker) greatly

facilitates the detection of "hot spots." IEC Publication 325 provides additional guidance on the uniformity

of probe response for alpha and beta contamination meters. Surface sensitivity measurements are also

discussed in this standard.

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