Fluke Biomedical 440RF - Manual (Page 22)

Victoreen 440RF/D
Operators Manual
2-14
External Background Radiation
Even in the absence of an artificial external radiation field, an external field exists due to building
materials which contain minute quantities of naturally occurring radioactive elements and due to cosmic
radiation. These two sources are termed background. Over any short space of time, they are static or
noisy because of the activities involved.
The average value of background ranges between 0.01 and 0.1 mR/h depending upon the location.
Because the instrument does not differentiate between background and any other low intensity source,
background must be indicated.
Internal Radiation
There is a less than 0.01 mR/h contribution due to trace quantities of radiation in the chamber materials
and due to residual scatter emitted by the check source.
Amplifier Leakage
Leakage current originating in the dual mosfet electrometer and in the shorting transistor comprise the
major source of fluctuation on the bottom ranges. It has been reduced to the lowest value possible. A
leakage correction routine in the software aids in reducing the effect of the leakage current.
Insulator Strain Charges
After rough handling and during transportation, the insulators in the probe assembly of the instrument
may have developed strain charges on their surfaces. If this has occurred, rest the instrument until the
charges bleed off. If the strain charges are upscale and relatively steady, a measurement can be made.
Then a non-zero compensation should be applied (refer to Non-zero Compensations).
As a preventative measure, subject the instrument to as little shock, vibration, and abrupt motion as
possible.
Statistics Of Radiation
Another limitation associated with the measurement of low-level radiation to any degree of accuracy, in
any short period of time, is the statistics of the source. Statistics refer to the idea that there is a lower
level of radiation flux per unit time intercepted by the chamber volume. Beneath this level, the radiation
flux is measured more as a series of individual photons and less as a smooth flow.
This effect can be observed when measuring the exposure rate of radiation sources on the 1 mR/h range.
For instance, the reading will fluctuate more frequently at 0.8 mR/h than it will when background only is
present. The amount of electrical filtration or signal averaging employed is a compromise between small
fluctuations and long response times.
Obtaining Readings from Fluctuating Indications
In order to differentiate background, the operational check source, and low intensity external radiation
from the slightly fluctuating display on the 1 mR/h and 3 mR/h ranges, some repeatable and unambiguous
method of evaluating the fluctuations should be employed. The following method is recommended.
While studying the fluctuations for approximately ½ minute, select an upper and a lower limit that most
(approximately 80%) of the fluctuations remain in. Mentally note or record these limits.
On positive (upscale) indications, disregard
occasional (5 to 30 mR/h) large fluctuations and
decays. These are believed to be cosmic rays or
chamber alphas.
Take the arithmetic mean, by adding the two values and dividing by two.
NOTE