Protean Instrument Corporation
120
Concepts MPC
900
Count
Time
Requirements
The counting time required to meet specific measurement sensitivity is dependent upon
the absolute counting efficiency, the inherent background of the instrument, and the
representative volume or mass of the sample. Measurement sensitivities are often
referred to as Lower Limits of Detection (LLD) or Minimum Detectable Activity (MDA)
and their respective equations were developed using statistical decision theory
techniques.
Low
Background
Material
Selection
Construction material used in the MPC-900 is carefully selected for low inherent activity.
The sample media used should also be selected carefully , if the lowest background
performance is desired. Stainless steel planchets are preferable to aluminum for this
reason.
Background
Stability
Now that most of the sources and remedies for background have been discussed, it is
appropriate to mention a few pitfalls. Backgrounds are due to random decay and
therefore are subject to statistical fluctuations. Backgrounds are due to random decay
and therefore are subject to statistical fluctuations. When background values are
determined, they are mean values gathered from several tests. The lower the
background becomes, the more difficult (time consuming) it is to determine the true
mean of the background value. Often the background fluctuations due to random decay
is confused as instrument instability.
A source of background instability is often attributed to the environment of the counting
room. A measurable amount of background can originate with radioactivity carried by
the ambient air in the counting room. This appears in the form of trace amounts of
radioactive gases or dust particles which settle on samples or the counting instrument.
Radon (Rn-222 and Rn-220) are short-lived radioactive gases which originate as
daughter products in the decay chains of the uranium and thorium present in soil and
laboratory construction materials. Their concentration in the atmosphere can vary
significantly depending on the time of day and meteorological conditions. Radioactive
dust can be largely eliminated through effective filtration (activated charcoal) of the air
supply to the counting room.
