Lab B1: Ionizing Radiation Safety

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.

II. References.

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,

1 gray = 1 siefert = 100 rads = 100 rems = 100 roentgens = 100 R's
Dosimeters measure dose. Survey meters measure dose rate, usually in mR/hr.

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/sec
A 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
This is a statistical statement - if one person gets 5000 R, he or she does not die of cancer! With any dose over 500 R, you are dead within a week from lack of white blood cells, always. But if 5000 people get 1 R each, one of them is likely to die of cancer, as a result of that radiation dose. This applies to integrated whole-body dose. This is a very hard number to measure, simply because of those 5000 people, about a third of them will die of cancer anyway. It would take a huge sample of people receiving 1 R each to measure this tiny increase in the cancer rate.

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
Below we will use these two numbers to discuss the concept of acceptable risk.

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?

V. Equipment
	Green survey meter: Technical Associates model PUB 1AB, with P-11 probe.
	Individual dosimeters:  XETEX model 415A
Appendix A. Doses Due to Ionizing Radiation
Natural background including cosmic rays and radiation from naturally radioactive material:
	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/yr
Natural background living in a:
	wood frame house		11 mrems/yr
	brick house			30 mrems/yr
Natural 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.
Appendix B. Handling of Sealed Radioactive Sources
The radioactive sources which we use in project lab are almost always sealed sources, in which the radioactive material is sealed inside a protective casing that prevents the material from leaving the source. These sources are quite safe; however, the following rules are to be observed while working with the sources:
  1. Remove a source from storage only when you are ready to use it.
  2. Do not eat, drink, or smoke in areas where sources are being used.
  3. Note in your lab book the type and activity of all sources used.
  4. Be certain that the sources are returned to the storage cabinet when you leave the laboratory.