11.2. Quantities Related to Dosimetry 605
Because the amount of radiation is an important factor in quantifying the possible
damage to materials, therefore the termsradiation exposureandradiation dosehave
been devised. It has been found that both living and nonliving things get affected
by not only high levels of instantaneous doses but also by low levels of sustained
doses. With the availability and use of high radiation environments, such as particle
accelerators, the question of radiation damage to electronics circuitry and detectors
is getting more and more attention. For example, a lot of work is being done
to produce the so called radiation hard silicon detectors, which could be used for
an extended period of time in high energy colliders. The present form of silicon
detectors become unusable after being exposed to high radiation fields for some
time (a couple of years) and must be replaced to continue the experiments. This is
a major drawback of these otherwise highly sensitive and dependable detectors.
A.1 Roentgen (R)..........................
Roentgen is a measure of exposure due to photons only (x-rays or gamma rays)
and is based on the amount of ionizations they produce in air. 1Requals the dose
needed to ionize and produce 2. 58 × 10 −^4 coulombs of positive and negative charges
in one kilogram of air. Since it is not applicable to all tissues, for example, bones,
and represents dose only due to photons, it is not used in dosimetry any more.
A.2 AbsorbedDose
The amount of energy deposited in a medium per unit mass of the medium by
ionizing radiation is calledabsorbed dose. It is measured in units ofJ/kg.For
absorbed dose the particular name of this unit has been chosen to begray(Gy)with
1 Gy =1J/kg.Apart fromGy, there is another unit of absorbed dose calledrad. Even though
radhas mostly been replaced byGyit is still found in some modern literature.Rad
was introduced in 1953 to replace Roentgen, which was the unit of exposure due to
x-rays orγ-rays only. It is defined as the dose equivalent to the absorption of 0.01
joule of energy per kilogram of tissue.
A.3 EquivalentDose
It is often quoted that absorbed dose is not capable of characterizing the biolog-
ical effect of radiation. That fact of the matter is, absorbed dose is not capable
of characterizing damage to any medium. All it tells us is how much energy has
been absorbed by the medium and not what this deposited energy has done to the
medium. In this respect absorbed dose treats all types of radiation equally. In other
words, for the case of absorbed dose, there is no difference between a photon and an
α-particle if they deposit the same amount of energy.
Hence when it comes to the effects of radiation, one can not use absorbed dose
as the relevant quantity. Since dosimetry is primarily concerned with the safety of
personnel, therefore a quantity calledequivalent dosehas been defined that charac-
terizes the damaging effect of radiation on tissues. The basic idea is fairly simple:
multiply the absorbed dose by a factor suited to the biological effectiveness of the