Exposure to Ionizing Radiation
Several experiments in the laboratory involve the use of radioactive materials which produce ionizing radiation. It is important that any student working with these radioactive sources have a good understanding of the nature of the radiations and of the maximum exposure levels that are permissible by federal and state law. Cornell University expects us to limit exposures to 10% of the maximum permissible. In this laboratory it is possible to keep exposures well below these levels and it behooves all of us to use sources wisely to minimize exposures to ourselves. To do otherwise would be very foolish indeed. The purpose of this summary is to present basic information about exposure levels and how they can be interpreted. Reference to more detailed information will be given at the end.
Properties of Radiation
Radiation from radioactive isotopes were named before they were identified and even today carry these historic names --alpha, beta, and gamma rays. Alpha rays are the nuclei of helium atoms and are characterized by their production of dense ionization over a very short path before coming to rest. Energies fall between 3 and 8 Me V, with corresponding track lengths in air from 2 to 7 cm. A few thousandths of an inch of any solid will thus absorb alpha rays.
Beta rays are positrons or electrons, again typically having energies up to a few MeV. Electrons scatter readily in matter and as a result do not have the sharp, well defined ranges that are characteristic of alpha rays. But roughly speaking they can be stopped by modest amounts of material such as 1/8" thick aluminum. Gamma rays are photons and so are identical to x-rays, except for their mode of production. Energies are again in the region up to a few MeV. Gamma rays penetrate sizable amounts of matter and their absorption is characterized by an exponential decrease in intensity with increasing thickness.
Measurements of the absorption of these radiations in matter are the subject of several P4410/6510 experiments. The problems associated with the use of sources of these radiations in the laboratory are quite different for the three types. Alpha emitters are generally used in an unsealed condition and care must be taken to avoid contamination which could lead to inhalation or ingestion. Instruments to detect contamination by alpha emitters are very delicate and must be capable of being placed directly in contact with possibly contaminated surfaces to avoid absorption by the intervening air. On the other hand, gamma sources are generally sealed in containers and their presence can readily be detected by relative]y simple equipment. Beta emitters fat] somewhere in between these extremes, and one finds them being used both sealed and open. In the P410/510 lab we use only sealed sources, which are periodically checked for leaks by Cornell radiation safety personnel.
Units of Radiation
We are "blessed" with a set of strange units which evolved from the earliest attempts to measure radioactivity. The important units are these:
curie: Originally defined as the number of disintegrations per second in a gram of radium. Now set equal to 3.7 x 1010 disintegrations/second and applied to any source.
- 1 Ci = 3.7×1010 dis/sec
- 1 millicurie (mCi) = 3.7×107 dis/sec
- 1 microcurie (μCi) = 3.7×104 dis/sec
becquerel: Bq = 1 disintegration/second
For Radiation Dose
roentgen: A unit of radiation dose. Originally defined as an amount of radiation that will release one electrostatic unit of charge (3.3×10-10 C) through ionization in one cubic centimeter of dry air at NTP ( or in 1.29×10-3 g of air). If we use the experimental result that fast charged particles expend on the average 34 eV in producing one ion pair in air and use the equivalence 1 eV = 1.6×10-12 erg = 1.6×10-19 J, the roentgen becomes an energy deposition of 87 erg/g or 0.0087 J/kg.
rad: Updated version of the roentgen, defined as an energy deposition of 100 ergs per gram of matter (0.01 J/kg).
gray: 1 Gy = 1 J/kg = 100 rad = 0.1 rem
For Dose Equivalent
rem: (roentgen equivalent man) Unit of dose designed to be relevant to biological effects. For beta and gamma radiation 1 rem = 1 rad in tissue. For any radiation 1 rem = 1 rad × QF where QF is called the relative biological effectiveness or quality factor. The QF is a number that is used to account for the experimental fact that very dense ionization produces more biological damage per unit of absorbed energy than light ionization.
sievert =1 Sv = 100 mrem
Some values of QF are:
- Beta and Gamma rays 1
- Thermal neutrons 4-5
- Fast neutrons 10
- Alpha particles 10-20
These units of dosage are measures of accumulated dose. We are interested in dose rates. So we will be using units such as rem/year, mrem/hour etc. Many radiation monitors are designed to read dose rated in rad/hour or mrad/hour. When we are dealing with gamma radiation in the lab such meters can be considered to read directly in rems as well as rads.
Maximum Permissible Exposure Levels
It is impossible to fix an exposure level below which it can be absolutely guaranteed that no damage to a biological system will take place. However, from a large amount of experimental evidence obtained during the past 50 years, exposure levels which are considered to pose no health hazard to an individual have been derived. Before writing down these levels, several interesting pieces of data can be noted:
- Background radiation. People at or near sea level are subject to a constant dose rate of about 150 mrem/year. About 25% comes from cosmic rays, about 10% is from the 40K in our own body, the rest from radioactivity in matter. This background level can be as much as three times higher for people living at the highest elevations.
- Lethal dose. A single, whole body, dose acquired in a short time (several hours) of 400 rem is known to have a 50 percent probability of being fatal. Known lethal doses vary from about 200 rem to 800 rem. These values show the influence of not only the state of health of the individual but also the kind of treatment given.
- Healing. The effects of radiation are cumulative over a period of time of the order of a month, but a healing process is evident in cases where the dose has been received over longer periods.
Maximum permissible occupational exposure levels are set by the Nuclear Regulatory Commission. They have decreased about a factor of ten from the time they were first set in 1934. The present levels for persons 18 years old or older working with radioactivity are:
- Whole body radiation -- 5 rem in a year
- Hands or feet -- 75 rem in a year
The federal and state maximum permissible exposures in a 3 month period (calendar quarter) are:
- Whole body, head and trunk~, active blood-forming organs, eyes or gonads --1.25 rem
- Hands and forearms; feet and ankles --18.75 rem
- Skin of whole body -- 7.50 rem
The levels in the second list may be exceeded if (I) the whole body dose does not exceed 3 rem in any calendar quarter and (2) the accumulated dose does not exceed 5(N-18) rem, where N is the person's age.
A simple interpretation of all these numbers is to say that one should not exceed a dose rate of 100 mrem per week. If you were working a 40-hour week in an establishment using radioactive sources you ( and your employer) would have to be sure that you were working in an area where the average dose rate was less than 2.5 mrem/hour. Cornell University recommends that exposure be limited to 10% of this dose rate.
Dose Rate Produced by Gamma Source
We can calculate the dose rate produced by a given gamma source. To see what factors enter into such a calculation, let us do an example to find the dose rate at a distance of one meter from a one MeV gamma source having a strength of one millicurie. We will assume that only one photon is emitted in each decay. We need to know the effective thickness of tissue in which the energy will be deposited. From the absorption coefficients for the light elements we can estimate that this length is 20 cm. Having this value, we can proceed:
Strength of source 1 mCi = 3.7×107 photons/second At 1 m, the intensity of radiation fa1ling on a surface is
3.7×107/4π × 104 = 2.9×102 photons/cm2s
With 1 MeV = 1.6×10-6 erg, the surface energy density is 2.9×102×1.6×10-6= 4.6× 10-4 erg/cm2s and if this is absorbed in 20 cm of tissue of density 1 g/cm3 , the volume and mass energy density becomes:
2.3×10-5 erg/cm3s or erg/gs = 8.4×10-2 erg/ghour.
Using 1 rad = 1 rem = 100 erg/gram we find the dose rate is:
8.4x10-4 rem/hour = 0.84 mrem/hour.
This quantity varies slowly with energy and since the photon energy of sources we use is not too different from one Me V we can use a rule of thumb, for a source that emits one photon per decay:
A one mCi gamma source produces one mrem/hr at one meter .
Significance for Physics 4410/6510
Before you can work safely with radioactive sources you must realize that time and distance play dramatic role in determining the dosage. A one milliCurie source at one meter poses no threat but if you were to carry the same source in your pocket for a few days you would have severe damage to the nearby tissue. But you could pick the source up in your hand for a few seconds without adding appreciably to your accumulated dose.
The sources in Physics 410/510 are described in an attached Appendix I. When you do an experiment in which you use a radioactive source you should measure the dose you receive in each lab session with a survey meter and record it in your notebook. You will find that with a little bit of care and planning you can keep the exposure comparable to the dose that you receive in the course of a few days from natural background radiation, far below the legal permissible exposure level.
(some library locations included)
- A. E. Profio. Radiation Shielding and Dosimetry (1979) TK 9152 P96
- P. A. Tipler, College Physics (1987). Section 32-5
- K. Z. Morgan and J. E. Turner. Principles of Radiation Protection (1967) RA 569 M84
- H. F. Henry, Fundamentals of Radiation Protection ( 1969) RA 1231 R2 H52
- Radiation Safety Manual. Cornell University, May 1981
- N. Y. State Sanitary Code Chapter 1. Part 16, Ionizing Radiation.
- Title 10 Code of Federal Regulation, Part 20: 10 CFR 20
- J. DeWire, January 1979
- Updated: E. Cassel, March 1989
- Updated: M. Tigner Apr. 1990