Johnson Matthey TRACERCO T401, User Manual, Page: 21 / 26

9 Monitor Response
The response of the monitor in counts / sec to a given level of surface contamination will depend on the
radiological and physical nature of the source being measured. This will include the presence of
daughter products in the decay scheme, thickness of the surface layer within which the activity is
deposited and the degree of backscattering which is taking place (influenced by the type of material
being monitored).
An ideal source would be one in which the activity is deposited in a thin layer at the surf ace wit h
m inim al self -absorpt io n or b ack scattering tak ing place. I n re alit y, s urf ace characteristics will
vary markedly and never achieve the characteristics of an “ideal” source.
In order to achieve traceability and accuracy Tracerco has calibrated the T401 for activity response using a
number of specified radionuclides in the form of large surface area plaques with a precisely
controlled radioactivity content. In normal use, it is unlikely that the physical characteristics of the
calibration source will match precisely those of the contaminated surfaces which are under
investigation. The relative impact that this will have on data interpretation will depend on the nature
of the radionuclide being determined.
These limitations should be borne in mind by operators carrying out any form of radiation
contamination monitoring and if there is any doubt, expert advice should be sought.
9.1 Affects of Gamma Radiation
The Tracerco T401 Contamination Monitor primarily responds to highly ionising alpha and beta radiation,
although gamma radiation will be detected with a substantially reduced efficiency. A simple check may be
carried out to determine whether the observed count rate is arising predominantly from alpha / beta radiation
or from gamma radiation. An absorber has been provided which can be placed between the source of
contamination and the sensitive volume of the detector. Any reduction in the observed counts per second
reading of the instrument will be primarily be due to absorbtion of alpha and beta radiation. Gamma radiation,
by comparison, will be relatively unaffected by the presence of the absorber
.