RADIATION
SAFETY TRAINING MANUAL
CHAPTER
2
UNITS FOR MEASURING IONIZING RADIATION
CHAPTER 2 Table Of Contents
A. ROENTGEN: THE UNIT OF EXPOSURE
B.
RAD: THE UNIT OF ABSORBED DOSE
C.
REM: THE DOSE EQUIVALENT UNIT
D. CURIE: THE UNIT OF ACTIVITY
When conducting a
survey of the laboratory for radioactive
contamination, you note that the instrument reads
in mR/hr (milliroentgen/hour). This is a
radiation exposure rate measurement. The
laboratory has another instrument which reads in
counts per minute (cpm). While opening a
radioactive vial, you notice that the label
describes the contents in microcuries. This is a
unit of radioactivity. When reviewing film badge
and finger ring records, you note that the
results are given in millirems. This is a measure
of radiation dose. These units, commonly used at
the University of California, San Francisco
(UCSF), are discussed below.
A. ROENTGEN: THE UNIT OF EXPOSURE
The roentgen (R)
was adopted in l928 as a unit of exposure to
medium-energy x-radiation. It is the approximate
exposure to one gram of radium located one yard
away for one hour (i.e. one gram of radium
produces an exposure rate at one yard of
approximately one R/hour). Specifically, the
roentgen is the quantity of x- or gamma rays that
produce 2.58 x 10- 4 coulombs/kg of
air at standard conditions of temperature and
pressure. In measuring the roentgen, a known
volume of air is irradiated, and the ions
produced (electrical charge) are collected and
measured. The choice of air as a standard
substance was for convenience. Since air and
water have an effective atomic number that is
nearly the same as that of tissue, absorption of
x-ray energy per gram of soft tissue, water and
air is within about 12% of being the same.
However, the
roentgen has limitations. By definition it is
limited to x- and gamma-rays, and medium of air,
and does not include other types of radiation.
Further, the definition of the roentgen holds
only for lower energy radiations (up to 3 MeV).
B.
RAD: THE
UNIT OF ABSORBED DOSE
The rad is the
unit of absorbed dose and is a measure of the
energy deposition per unit mass by all types of
ionizing radiation. Chemical and biologic changes
in tissue exposed to ionizing radiation depend
upon the energy deposited in the tissue rather
than the amount of ionization which the radiation
produces in air. The rad, an acronym for Radiation
Absorbed Dose, is not
limited to x- or gamma rays and is not limited to
the medium of air.
The rad is
specifically defined as the deposition of 100
ergs per gram of absorbing material. As a general
rule, the absorbed dose in soft tissue from 1 R
of intermediate energy x- or gamma rays is about
1 rad. The rad is being replaced by the Gray
(Gy), which is defined as an absorbed energy 100
times greater than a rad (1 Gy = 100 rad = 1
joule/kg). This Manual has retained the older
units of rad, rem, curie.
C.
REM:
THE DOSE EQUIVALENT UNIT
The rem, an
acronym for Roentgen Equivalent
Man, was developed in response to
evidence that biologic effects per rad of various
radiations are often different. The dose
equivalent (DE) is defined as the absorbed dose
(rads) multiplied by a quality factor (QF), a
term that expresses the differences in biologic
effectiveness of various types of radiation as
compared to x-rays. The QF is a function of the
linear energy transfer (LET) of the radiation.
The QF for x-rays, gamma-rays, and beta particles
with a maximum energy of greater than 30 KeV is
1.0. This category represents a majority of
radioactive materials used at UCSF. For
information, the QF for neutrons and protons with
energies less than 10 MeV is 10 (30 for
irradiation of the eyes); for alpha particles
from natural radionuclides the QF is 10.
The new unit for
dose equivalent is the Sievert (Sv), which is
related as 1 Sv = 100 rem.
TO SIMPLIFY
THIS MANUAL AND TO MAKE CALCULATIONS EASIER, THE
TERMS ROENTGEN, RAD, AND REM ARE CONSIDERED
INTERCHANGEABLE.
D. CURIE: THE UNIT OF ACTIVITY
When an excited
nucleus emits characteristic neutrons, alpha,
beta (positive or negative) particles, and/or
gamma rays, the nuclei are said to be
radioactive. (Radioactive materials used at UCSF
primarily emit beta particles and gamma rays.)
Each transformation of a parent nucleus is called
a disintegration. Cobalt-60, often used in
radiation teletherapy, emits a beta particle
followed immediately by two gamma rays. These
three radiations are emitted per disintegration.
An important unit
in the practical application of radioactivity is
the number of disintegrations per unit time
(typically seconds or minutes). The quantity of
any radionuclide in which the number of
disintegrations per second is 3.7 x 1010 is
one curie (Ci).
- 1 Ci = 3.7 x
1010 disintegrations per
second (dps)
A millicurie is
one-thousandth of a curie and microcurie is
one-thousandth of a millicurie. The curie is
being replaced by the becquerel (Bq) unit defined
as: 1 dps. Thus
- 1 Ci = 3.7 x
1010 Bq
- 1 Bq = 2.7 x
10-11 curies.
Units of
conversion are found in the Glossary.
Many UCSF survey
meters read in counts per minute (cpm) or counts
per second (cps). If the counting efficiency
(counts per unit time/disintegrations per unit
time) is known for the radioactive material being
measured, then the activity of the material can
be estimated. The efficiency will vary for each
isotope and instrument type.
Assume that the
efficiency of a survey meter for measuring 35S
is 1%. Assume that 1,000 cpm are measured with
the meter. Then, dpm would be computed as
1,000/0.01 or 100,000 dpm. The activity would be
computed as 100,000 dpm/60 sec/min = 1,667 Bq or
0.045 microcuries.
|