Page 538 - Instrumentation Reference Book 3E

introduction 521

(Mii) is added to the basic LiF. The advantages of rads) per year. Hence, while it is almost impossi-
EiF are: ble to seduce radiation exposure to zero il is
impo~~ant to ensure that using a radioactive
(1) Its wide and linear energy response from
30 KeV up to and beyond 2 MeV; source does not increase the dose to a level
(2) Its ability to measure doses from the mR to greater than many other hazards commonly niei
in daily life.
105R without being affected by the rate at There are three main methods for miiiirnizing
which the dose is delivered-”-tliis is called the hazards due to the use of a radioactive source.
“dose-rate independence;” There are:
(3) Its ability to measure thermal neutrons as
well as X-rays, gamma rays, beta rays, and (1) Shielding: a thickness of an appropriate
electrons: material. such as lead, should be placed
(4) Its dose response is almost equivalent to the between the source and the worker.
response of tissue, Le., it has almost the same (2) Distance: an increase in distance between
response as the human body; source and worker reduces the radiation
(5) It is usable in quite small amounts, so it can intensity.
be used to measure doses to the fingers of an (3) Time: the total dose to the body oftke worker
operator without impeding the operator‘s depends on the length of time spent in the
work; radiation field. This time should be reduced
(6) 11 can be re-used many times? so it is cheap. to the niin~mu~i necessary to carry out the
required operation.
Another phosphor which has become quite
popular in recent years is calcium fluoride with The above notes are for sources which are
,
~
:
n
~
manganese ~ ~ a ~ which has ~been found tu contained in sealed capsules. Those which are in
be more sensitive than LiF for lox7-dose measure- a form that might allow them to enter the body’s
ments (some ten times) and can measure a dose of tissues must be handled in ways which prevent.
1mR yet is linear in doserate response up to such an occurrence (for example, by operating
10’R. However, it exhibits a large energy depend- inside a “glove box,” which is a box allowing open
ence and is not linear below 300 KeV. radioactive sources to be dealt with while the
Thermoluminescence has also been used to operator stays outside the enclosure). Against
date ancient archaeological specimens such as internal exposure the best protection is good
potsherds, furnace floors, ceramic pots, etc. This housekeeping, and against external radiation the
technique depends on the fact that any object. best protection is good instrumentation, kept in
heated to a high temperature loses inherent operating condition and USED.
thermoluminescent powers and. if left for a long Instruments capable of monitoring radioactive
period in a constant radioactive background, hazards depend on the radiation or particles to be
accumulates an ainount of TL proportional to monitored. For gamma rays, emitted by the most
the time it has lain undisturbed in that environ- usual radioactive sources to be handled. a variety
ment. of instruments is available. Possibly the cheapest
yet most reliable monitor contains a Geiger coun-
ter, preferably surrounded by a suitable metaY
22-13 ~~~~t~ and safety
covering to modify the counter’s response to the
who
~n~~ne works with r~dioactive inateriafs vasied energies from gamrn~-emi~~~ng radioiso-
must ~~nders~and clearly the kinds of hazards topes to make it similar to the response of the
involved and their magnitude. Because radio- human body. There are many such instruments
activity is not directly observable by the body‘s availabk from comnniercial suppliers. More elab-
senses it requires suitable measuring equipment orate ones are based on ionization chambers,
and handling techniques to ensure that any expos- which are capable of operating over a much wider
ure is minimized, and, because of this, suitable range of intensities aid are correspondingly inore
legislation governs the handling and use of all expensive. For beta emitters, a Geiger counter
ra.dioactive material. In Part 4 an outline of the with a thin window to allow the relatively easily
regulations is given as well as advice on contact- absorbed beta particles to enter the counter is
ing the local factory inspector before the use of again the cheapest monitor. More expensive
any radioactive source is contemplated. monitors are based on scintillation counters
Because everyone in the world already receives which can have large window areas, useful for
steady radiation (from the natural radiopotas- monitoring extended sources or accidents where
sium in the human body and from the general radioactive heta emitters have been spilt. Alpha
background radiation to which all are subjected) detection is particularly difficult: as most alphas
the average human body acquires a dose of about are absorbed in extremely thin windows. Geiger
300 micro-grays (pGy) (equivalent to 30 niilli- counters with very thin windows can be used, or