None -Reply

[FRAMEP@ORAU.GOV]

Joe Kane wrote:

>I'm looking for a NuReg reference or other for MDA (Minimum
>detectable activity) Calculations for portable survey meters.   If
>anyone know of any please post.

I'll send you a list but here are a couple.

1. "Proper Selection and Application of Portable Survey Instruments
for Unrestricted Release Surveys" by Ed. Walker presented at 1994
Int. Symp On D&D, Knoxville TN Apr 24-29

2. Draft Nureg CR-5849, ORAU-92/C57 "Manual for Conducting
Radiological Surveys in Support of License Termination"

There are two situations: scans and static measurements.

Scans. In a scan it is the surveyor's ability to analyze the audible
signals that is the limiting factor.  NUREG 5849 gives the following
rough approximation.

MDA  =  100R/EA

where R is the minimum discerable audible increase in countrate cpm,
E is the detector efficiency(cpm/dpm) and A is the probe area in sq
cm. The MDA is dpm/ 100 cm sq. Note that the physical area of the
probe, not the effective area, should be used.

The NUREG states that "experience has shown that a 25 to 50% increase
can be easily identifiable at ambient background levels  of several
thousand cpm [eg NaI probe] whereas at ambient levels of a few counts
per minute [eg ZnS] a two to three fold increase is required"

Bishop has suggested the following:

MDL = 100 B/EA where B is the background countrate. The assumption is
that a doubled background can be detected over a one second period
during which the detector is over the source. In other words a GM.

Walker (who assumes we are watching a bouncing needle) suggests a
minimum sensitivity be calculated as follows:

MS  = [4.65 (B/2t)*0.5]/ [ E(A/100)] 

where t is the ratemeter time constant (usually 2 to 22 seconds).
Since the effective efficiency of the detector is reduced by moving
the probe Walker calculates it by multiplying the efficiency measured
statically by 1 - e*-(d/vt)
where  d is the length of the detector window in the direction of
travel eg cm and v is the probe velocity in cm/sec 

For static measurements with survey meters the standard formula is

MDA = [4.65(B/2t)*0.5]/[E(A/100)] 

Best of luck

Paul Frame
Oak Ridge Assoc Univ.