Radiation_safety

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Radiation Safety - S2004
The consequences of excessive radiation exposure are profoundly bad. Do your
homework and think carefully about what you are doing.
Ionizing radiation is typically undetectable with human senses. Use survey
instruments and make careful estimates to keep dose levels low.
Common sense:
1) Know the nature and strength of sources with which you are working. Estimate
dose rates for your use conditions before you start the lab.
2) Maintain distance from sources (use tongs, sit a minimum of a meter away).
3) Check your vicinity with a survey meter.
4) Use shielding when appropriate.
5) Minimize time near sources.
6) Do not eat in the lab.
7) Wash hands when you leave the lab. Use gloves when working with unsealed
sources.
8) Never remove sources from the lab.
Information sources:
Level and decay schemes, radiation type, dose programs
http://www.nndc.bnl.gov/nndc/nudat/
Units and conversion factors, exposure limits, and protection procedures
http://pdg.lbl.gov/2000/radiorpppbook.pdf
Radiation units:
Curie (old unit of activity)- 3.7x1010 disintegrations per sec.
Bequerel (new units of activity) - 1 disintegration per second
Flux - the number of quanta incident on a 1 cm2 area per second.
Roentgen - the amount of gamma radiation that produces 1 esu of ionization in 1
cm3 of air.
Rad (old unit of absorbed dose)- that amount of radiation that produces 100 ergs
in each gram of tissue it traverses.
Gray (Gy - new unit of absorbed dose) - 1 J/kg( = 100 rads).
rem - (old unit of biological efficacy x dose) - that amount of any type of radiation
that produces the same biological effect as 1 roentgen of gamma radiation.
Sievert (Sv - new unit of biological efficacy x dose) - 1J/kg (= 100 rem).
For gamma rays, the Roentgen~rad~rem and Gray~Sievert.
Some Radiation Dose Limits to Members of the General Public
100 mrem (1 mSv) per year total effective dose equivalent to the whole body
2 mrem (0.02 mSv) total effective dose equivalent to the whole body per hour
500 mrem (5 mSv) total effective dose equivalent to the fetus of a Declared Pregnant
Woman per gestation
Practical approaches to estimating and limiting dose:
1.
Check the nature and strength of the source you will be using. For example it
will read: Cs137  (661 keV) 1 Ci.
 Look up the conversion from energy dose to biological efficacy (Grays to
Sieverts, or rads to rems). (For gamma rays it is ~ 1.)
2. Conduct a Geiger counter survey of the area where you will be working.
 If the signal exceeds 1 mrem/hour, contact the professor.
3. Make sure the film badge area monitor is near where you will be working.
 This is only a measure "after the fact."
4. Inspect the source. Is it sealed? (If so, danger of ingestion of powder fragments
is low.)
 Is the source sealing intact? If not, contact the professor.
5. Estimate the biological dose to an experimenter carrying out a typical experiment
with the unshielded source.
 Typically you should estimate a worst case scenario of 10 hours at 1 meter
from the source.
 A useful conversion for gamma rays: 1 Curie of gamma source
produces ~ 1.3 rem per hour at a distance of 1 meter.
6. Compute the effect of any shielding that is present.
 The intensity of a beam passing through matter is given by
I
 e  x  e  
I0
where

is the absorption coefficient, x is the
 is the mass attenuation coefficient for rays of that energy
and shielding, and  is the amount of shielding in g/cm2.
path length,
Homework 1, Due Friday Jan 17, 2004
1)
A Na22 laboratory source is a 511 keV gamma ray emitter. It has a labeled
strength of 1 Ci. What is the dose to a person who sits at one centimeter, one
meter, or two meter distance for 10 hours? (Do three calculations.)
2)
The following table gives the absorption coefficient in cm-1 for gamma rays in
various materials.
Energy (keV)
Water
Iron
Lead
10
4.88
1330
1340
100
0.165
2.6
58.2
1000
0.07
0.47
0.78
Estimate the thickness of each material necessary to decrease a Na22 gamma dose by a
factor of 1000.
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