Fluke Biomedical 90-12 - Manual (Page 10)

Victoreen 90-12
Operators Manual
2-2
2.4 Use in Adverse Conditions
Because of its ruggedness, wide temperature range, and the fact that it is moisture resistant, the probe
may be used in a variety of adverse conditions. For example, the probe may be used out of doors at any
time of the year and even in the rain and fog. It is NOT recommended, however, that the probe be
submerged.
2.5 The Beta Shield
With the beta shield in the closed position, that is covering the open window, the probe measures only
penetrating radiation such as x-rays and gamma rays. In this configuration the probe will not respond to
beta rays or alpha particles.
The probe's windows may be opened by gently twisting and pulling both shield halves apart. When each
half has reached its limit of travel the windows are maximally exposed. In this mode the probe will
respond to beta rays, x-rays and gamma rays.
Measuring a particular location with and without the shield in place can provide information about the
nature and type of radiation present. If beta rays or very low energy x-rays are present then the readings
with the beta shield open will yield higher readings than the readings with the shields closed.
2.6 The Open Windows
There are two windows opposite each other on the barrel of the probe. This area is the central and most
sensitive part of the GM tube. Each window subtends 90 degrees of arc. Together they subtend 180
degrees or 2 radians. This permits maximum independence of directionality. For best results, when
surveying for beta rays on surfaces, the operator should orient the probe so that one of the open windows
points directly at the surface.
2.7 Error Analysis
From the sensitivity of the probes and the Gaussian distribution statistical model for radioactive decay
measurements, the measurement error can be calculated directly as a function of exposure rate and the
counting time for each probe. For this distribution the standard deviation is the square root of the number
of counts (σ=√ Total Counts). The total number of counts is equal to the exposure rate times the tube
characteristic times the total counting time (Total Counts = Exposure Rate x Tube Characteristic x Time).
The total counts is the mean or expected measurement.
It is also given for gaussian distributions that 99.73% of the actual measurements will fall within the mean
plus or minus three standard deviations (μ ± 3 σ). This is then chosen as the error range.
From the above discussion the accuracy of any probe can be computed.
2.8 Minimum Detectability
Minimum detectability depends on local background levels and, in the presence of a radiation
background, is usually calculated at the 50% and 95% confidence level. A good first approximation to
each is as follows: First, determine the mean and standard deviation of the background. Look up the
sensitivity of the probe to the nuclide. Factor in any geometry efficiency. The 50% confidence level is the
activity level, accounting for geometry and sensitivity, of the nuclide at the mean plus 3.0 standard devia
tions of the background. For the 95% confidence level use the mean plus 4.645 standard deviations of
the background.