In this lab you will learn how to measure ionizing radiation exposures and to survey for sources of radiation. You will learn the official radiation exposure limits and will set your own personal standard for radiation exposure.
I. Prerequisites. None. This lab is a prerequisite to all other labs that use ionizing radiation.
Particle Data Group MS 50-308 Lawrence Berkeley Laboratory Berkeley, California 94720Request the full package, including booklet, diary, maps, and plastic wallet.
III. Definition of Terms. We will give here a few rough, rule-of-thumb definitions of some important quantities, just to get you started. More precise definitions of these terms are given in the references.
Absorbed radiation dose - R's, grays, etc. Roughly speaking, absorbed dose is a measure of the total energy deposited by radiation in water or human tissue. To a crude approximation,
Activity of a source; curies, becquerels. Activity measures the number of radioactive decays in a sample per second.
1 Ci = 3.7 x 1010 decays/sec 1 Bq = 1 decay/secA typical source activity is one mCi.
IV. Personal Radiation Tolerance. Consult the references above to inform yourself on the biological effects of radiation, and on the major sources of radiation that everyone is exposed to. Melissinos (p. 147) gives an excellent summary of the effects of large doses of radiation. However, to that list should be added one fact, not available when his book was written:
|5000 person-R (whole-body dose) = one cancer death|
It is instructive to calculate a second figure for comparison. This is the number of person-miles driven per traffic accident. Californians drive about 150,000,000,000 miles a year, with about 4000 traffic deaths. Dividing these two numbers gives the result:
|37,500,000 person-miles = one traffic death|
A. Effect of Natural Background. As an instructive exercise, calculate the probability that you will die of cancer due to exposure to the natural radioactive background of 100 mR/year. This is an approximate calculation - don't worry about factors of two.
B. Personal Radiation Tolerance. Now decide on (and record in your lab book) your personal upper limit for the integrated exposure that you are willing to receive from man-made sources
(1) in a year; (2) in a lifetime; (3) in one semester of project lab.Also determine
(4) the exposure rate, in mR/hr, which you should consider alarming when indicated on a survey meter or personal dosimeter.Consult with peers and the instructor in reaching these decisions.
C. Radiation Survey. Having established a reasonable and prudent upper exposure level, it is now time to survey the environment. Familiarize yourself with the personal "chirping" dosimeters and the nuclear survey meter. Using the survey meter, determine the background radiation level outside where it is green and peaceful, in the halls of Thornton and in the middle of room 124. Are all these levels safe? You may get a better measurement with the dosimeter, since it measures mR's. Natural background is about 13 chirps per hour.
Now look for the hot spots. Use the survey meter to explore every crack and corner of room 124. There are at least two hot spots. How hot are the hottest areas? What is the source of radiation? Is it a hazard in its current form? What is the maximum time you can spend in the vicinity of these sources without receiving a significant exposure?
Green survey meter: Technical Associates model PUB 1AB, with P-11 probe. Individual dosimeters: XETEX model 415A
San Francisco 120 mrems/yr New York 135 mrems/yr San Diego 170 mrems/yr Denver 300 mrems/yr Pocos de Caldos, Brazil 7,000 mrems/yrNatural background living in a:
wood frame house 11 mrems/yr brick house 30 mrems/yrNatural radioactive materials in the body:
K-40 Whole body 17 mrems/yr K-40 Brain 30 mrems/yr Rb-87 Whole body 0.69 mrems/yr Rb-87 Pancreas 2.8 mrems/yr Ra-226 Whole body 2.8 mrems/yr Ra-226 Bone 28 mrems/yr U-238 Kidneys 1.2 mrems/yr Rn-222 Lungs 200-1,100 mrems/yr (depending on location & housing) Two-week vacation in the mountains 3 mrems/yr (due to an increase in cosmic rays at higher elevation) Cross-country jet flight >1 mrem/hr Radium-dial watch (dose to wrist area) 10 mrems/day Fallout from nuclear test 5 mrems/yr Whole-body diagnostic X-ray up to 25,000 mrems Chest X-ray up to 100 mrems/film Complete dental X-ray up to 5,000 mrems NRC, NCRP & EPA whole-body limit Occupational 5,000 mrems/yr General Public 500 mrems/yr Living at boundary fence of nuclear reactor <5 mrems/yr (Except Chernobyl, Three-Mile Island, etc.) Levels of Radioactivity in Liquids (Picocuries/Liter) Water discharged from nuclear power plant 1-10 Domestic tap water 20 River water 10-100 Beer 120 Ocean water 350 Whiskey 1200 Milk 1400 Salad oil (16 ppm C-14/gm of carbon) 4900 from: University of California, San Diego, Environmental Heath and Safety.