| dc.description.abstract | Measurement of the external gamma radiation was done at various radiological facilities
in Nairobi using readily available field survey technique and instruments. Film badges
were placed at various places where man made radiation sources are used and also where none other than the normal background radiation is present. After periods varying from
two days and extending up to two months of continuously monitoring for ambient gamma
radiation the films were processed and analyzed for dose recorded on them. Performance
checks and stability evaluations were done for equipment used to measure exposure
during calibration exercises for selected control films which were exposed to known
radiation.
Radiation exposure for various occupational categories was estimated in particular those
of radiographers working at Kenyatta National Hospitals' Radiotherapy and X-ray
Departments, physicists and technicians working at the Physics Section of Materials
Testing and Research Laboratories' and also researchers and technicians working at the
Institute of Nuclear Science. The deduced exposure was converted to dose using
conversion factors available in relevant references and data obtained from measurements
of radiation beams using dosimetry instruments. Data obtained showed that the doses
received by radiographers and other personnel of Kenyatta National Hospital were the
least as they averaged at 0.4 millrSievert (mSv) per month, This was as expected
considering the x-ray equipment and the radiotherapy sources are built in radiological
safe structures. In contrast the dose rates on the surface and I meter distance from 200
liter drums, containing spent sealed sources of radioactive materials could result in an
individual receiving as high as 10 mSv per year. The latter used to be accessible to staff
involved with radiation work as well as members of the public since the drums were
stored in the open air within the compound.
Many radiation workers are not monitored for occupational exposure. This was found to
be the case among the least enlightened group of workers such as those found in industry but also researchers and technicians who were well versed in radiation protection
principles. The dose received by the exposed personnel at the Institute ofNuc1ear
Science was found to vary considerably depending on what type of sources are in use and
also how frequently they were used. In one month it would have been possible for a
researcher or technician working up to eight hours a day to receive more than 5 mSv but
when the workload was low one would receive less than I mSv.
Results of cross checking the reliability of ionizing chambers and electrometer belonging
to the Radiation Protection Board by comparing them to those of the International Atomic
Energy Agency and Radiotherapy Department ofKenyatta National Hospital are
presented. The ion chambers, electrometers, cobalt source gamma beam and dose rate
meter did not give values which deviated outside measurement uncertainties during the
whole study period. In particular, the ion chambers were stable and the values obtained
when comparing the calculated dose to that they measured shows the accuracy was less
than the upper limit of deviation (± 5%) allowed for radiotherapy work. The stability of
the dose rate output from the cobalt teletherapy unit also gave a dosimetry accuracy of
better than ±5%.
One main limiting observation on film badge use was that in humid conditions noted for
some months, the films were not reliable when left for periods exceeding one month in
environments modified by man made radiation. Also for cases where an appreciable dose
was recorded, especially a single dose above 10 mSv, they could not be evaluated reliably
since the deviation would be as high as ± 100% between any two films placed at the same
site and presumably had received the same dose. Also for values below 0.4 mSv the film
badges could not really distinguish the dose received since this bordered on their
detection limit. Despite these shortcomings, the use of film badge dosimetry for
measurement of radiation exposure would offer assurance that workers are or are not
being over exposed when working in environments modified by man made radiation. | en |
