THE EFFECTS OF TIME, DISTANCE AND SHIELDING ON RADIATION
EXPOSURE
Purpose:
This lab is designed to illustrate the effects of time of exposure, distance
between source and worker, and type of shielding used of the dose a
radiation worker receives during an experiment.
Objectives:
1. To examine how the length of time a worker is exposed to a radioactive
source affects his dose.
2. To illustrate how the dose a radiation worker receives changes
as her distance from the source varies.
3. To examine how different types of shielding provide different levels of
protection from radiation exposure.
4. To learn how alpha, beta and gamma radiation vary with respect to
time, distance, and shielding characteristics.
Introduction:
Radiation workers employ three safety factors to minimize their
exposure when working with radioactive materials. First, by minimizing
the TIME that they are exposed to a source of radiation, they greatly
decrease the dose they receive. This means that, when working with
radioactive materials, workers carefully plan and execute experiments so
that they can move quickly through the procedure and, simultaneously,
can avoid the need to unnecessarily repeat experiments. Workers will
ALWAYS have a detailed procedure written out beforehand, and will
often perform a “dry run” with non-radioactive materials in order to assure
the experiment will run smoothly.
Workers will also minimize their exposure by keeping the
maximum possible DISTANCE from the source of radiation. As the
distance between the worker and the source increases, the radiation dose
received decreases. This is intuitive. However, the inverse square law
states that the dose received decreases with the square of the distance from
the source. For example, if a radiation worker DOUBLES her distance
from a source, her dose is now ONE-FOURTH of the original dose.
Finally, workers must often work in close contact with radioactive
materials. In these cases, the workers are protected by different types of
SHIELDING. The appropriate material to use as a shield depends upon
the type of radiation being produced. Alpha particles do not present an
external hazard. Therefore, no extra shielding is required, although
workers will still wear lab coats, glasses and gloves. Beta particles may
have much more penetrating power than alpha particles and so workers
will use a shield to absorb the energy released from these particles.
Generally, a one-quarter inch thick Lucite shield will provide adequate
protection. Note that lead, although very dense, is not a good choice for
beta particles because, as the particles release their energy into the lead
shield, X-rays are produced. Gamma rays and X-rays are the most
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penetrating forms of radiation and require the most dense shielding.
Concrete or lead are general used as shields for these materials. If a
source produces both beta and gamma radiation, a composite shield of
lead and Lucite may be used. Such a shield will greatly reduce beta
radiation, reduce gamma ray penetration by ten-fold, and not produce any
X-rays.
STUDENTS MUST WEAR GLASSES, GLOVES AND LABCOATS!!!!
Procedure:
I.
The effect of time on radiation exposure
a. Obtain an alpha radiation source. Place the detector window 1 cm from
the source. Determine the cpm. Record this value.
b. Obtain a beta radiation source. Place the detector window 1 cm from the
source. Determine the cpm. Record this value.
c. Obtain a gamma radiation source. Place the detector window 1 cm from
the source. Determine the cpm. Record this value.
d. Look on the side of the radiation detector. Record the rating of the
detector for each of the different types of radiation.
II.
The effect of distance on radiation exposure
a. Obtain an alpha radiation source. Place the detector window 1 cm from
the source. Determine the cpm. Record this value.
b. Move the alpha radiation source to a final distance of 2 cm from the
detector window. Determine the cpm. Record this value.
c. Move the alpha radiation source to a final distance of 4 cm from the
detector window. Determine the cpm. Record this value.
d. Move the alpha radiation source to a final distance of 8 cm from the
detector window. Determine the cpm. Record this value.
e. Move the alpha radiation source to a final distance of 16 cm from the
detector window. Determine the cpm. Record this value.
f. Obtain a beta radiation source. Repeat steps a through e.
g. Obtain a gamma radiation source. Repeat steps a through e.
III.
The effect of shielding on radiation exposure
a. Obtain an alpha radiation source. Place the detector window 4 cm from
the source. Determine and record the cpm value.
b. Place a Lucite shield between the source and the detector window.
Maintain the 4 cm distance between the source and the window.
Determine and record the cpm value. Record the thickness of the Lucite
shield.
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c. Repeat step b with additional thicknesses of Lucite shield if possible.
d. Place a lead shield between the source and the detector window. Maintain
the 4 cm distance between the source and the window. Determine and
record the cpm value. Record the thickness of the lead shield.
e. Obtain a beta radiation source. Repeat steps a through d.
f. Obtain a gamma radiation source. Repeat steps a through d.
BEFORE LEAVING LAB, USE THE RADIATION DETECTORS TO CHECK
HANDS, BODIES AND FEET FOR CONTAMINATION. ALSO CHECK YOUR
WORK SURFACE, PENCILS, AND LAB NOTEBOOKS. Because you are using
sealed sources, there is no real risk of contamination. However, whenever working
with radiation, a worker will check himself and work area for contamination prior
to leaving the “hot area”.
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