Interpreting Readings

Health physics, the field that pertains to radiation and its effects on man, is very complex, and theories and conclusions are constantly being updated as information becomes available. Data from occupational exposure, animal studies, and events like Hiroshima and Nagasaki have fairly well established the maximum safe exposure limits for man. Whether low level radiation causes cancer and birth defects is still being debated. Delayed effect, which could take years to develop, is difficult to study and therefore, there are no well-defined lower limits on ionizing radiation. Two publications entitled "Hormesis with Ionizing Radiation," 1980 and "Radiation Hormesis," 1991 (CRC Press, Boca Raton) present over one thousand examples of statistically valid data showing no physiological harm in vertebrates from whole body exposures to low dose radiation (<20mgy/y). 

The units mR/hr (milli-Roentgen per hour, or 1/1000th of a Roentgen per hour) pertain only to gamma radiation. Often other units of measurement similar to mR/hr are used. The term "REM" (Roentgen Equivalent Man) includes the effects of beta, alpha and neutron radiation. Measurement in REMs is more complete as radiation affects man, but such measurements are a complicated combination of many measurements each made with specialized detectors. 

It is important to note that the field intensity from a radioactive object decreases very quickly with distance. 

If the object is very small, increasing the distance from the object by a factor of two decreases the radiation level by a factor of four. This is called a square law situation, which demonstrates the dependence of proximity on dose for small radioactive sources. Larger sources, such as a large deposit of radioactive minerals, will show much less of this effect. In trying to estimate the danger of radioactive materials, it is important to take into account many aspects of the situation. For instance, the radiation level at the face of a radium-dial watch may be 3mR/hr, but the measurement taken from the back of the watch may be 0.3mR/hr. 

Another interesting point concerns the energy of the radiation. Geiger Counters will register one click whenever they detect a ray or particle of radiation hitting them. These tiny high speed bundles of energy are like short bursts of light. Some are extremely energetic, while others are not. Geiger Counters cannot determine the energy of the impinging ray, they only detect its presence. Sper Scientific models 840007 and  840026  detect Beta and gamma radiation starting at approximately 30KeV and up to 1.5 MeV. 

The opposite is the case for cosmic rays, which have enormous energy—some millions of times more energetic than anything found here on earth. The compensation figure for radiation of this type is difficult to estimate, due to the extreme range of energies that have been measured. 

Radiation - What is it?

Nuclear physics is a very complex field; however, the basic principles can be simply explained. 

All matter is composed of atoms. Atoms alone and bonded together in molecules form all the things around us, including ourselves. These atomic units are extremely small; so small, in fact, that a single grain of table salt contains approximately 1,000,000,000,000,000,000 atoms (this is not a misprint). It is impossible to see an atom, except with a sophisticated electron microscope, so many of our present day theories on the structure and composition of single atoms are based largely on the study of radiation given off from unstable (radioactive) substances. 

Atoms are composed of three basic particles: protons, neutrons and electrons. Electrons are extremely light, negatively charged particles that exist as a cloud around the center, or nucleus, of the atom. Sometimes the electrons are said to occupy orbits around the nucleus. These electrons are attracted to the nucleus because of the positively charged protons that, along with the neutrons, make up the nucleus. Atoms bond together in molecules when one atom gives up or shares an electron with another atom. Chemical reactions utilize this bonding process. 

In all atoms, the number of electrons (and therefore the number of negative charges) equals the number of protons (positive charges). The number of protons or electrons in an atom determines the chemical nature of the atom, and each element has its own unique number (example: hydrogen=1, helium=2, etc.). The number of neutrons, however, may not always be the same in every atom of a particular element. Atoms of an element with different numbers of neutrons are called isotopes. Every atom of a particular element has the same atomic number, but different isotopes of a given element have different atomic weights. 

It is the variable number of neutrons in the nucleus of an atom that leads to a process called nuclear decay that causes radiation. When an atom has too many or too few neutrons in its nucleus, it will have a tendency to rearrange itself spontaneously into a new combination of particles that are more stable. In this decay process, bundles of excess energy are shot out of the nucleus in one of a number of ways. 

When the neutrons are excessive, a neutron can convert itself to a proton and shoot out an electron at very high speed, known as beta radiation. 

A proton may be converted to a neutron to cause an unusual particle called a positron to be ejected from the nucleus. 

In still another process, the nucleus, in a vain attempt to stabilize itself, kicks out two protons and two neutrons all together as one particle, called an alpha particle. 

The energy released in each decay can be enormous. This decay process is utilized in atomic reactors and bombs. When certain very heavy isotopes of uranium or plutonium (or several other isotopes) decay, they may split apart. This process is called fission. In fission, the entire nucleus splits apart, causing two new atoms and releasing a very large amount of energy. This process is not very predictable, for the nucleus can split in many ways, yielding a wide variety of new atoms and even some free neutrons. The free neutrons, when released, can be absorbed by other fuel atoms, causing them, in turn, to fission—leading to a continuous or (if not controlled) explosive chain reaction. Due to the wide range of new atoms produced in the fission process, many of the daughter products are not stable and will, in turn, decay themselves, leading to hazardous nuclear waste and fallout. 

In all of the above processes, another kind of radiation, gamma, is almost always released. Unlike the particles previously mentioned, gamma radiation consists of tiny discrete bundles of energy called quanta. Light, X-rays and gamma rays can all be described as quanta, the difference being the total energy packed into each bundle. 

In nuclear decay some energy in the unstable nucleus is dissipated to its surroundings in the form of heat and radiation in the instant that it decays. The nucleus may remain in its unstable state for billions of years, and then suddenly decay spontaneously. The time required for half of the atoms of a particular isotope to decay is called the half-life of that isotope. For an isotope with a half-life of one year, the pure isotope substance would be only 50% pure after one year, half of the original atoms having decayed into some other substance. After another year, 25% of the original material would remain, and so on. Natural radioactive materials in our world are only those with very, very long half-lives. Uranium-238, for example, has a half-life of 4 billion years, and exists today only because not enough time has elapsed since its creation for it to decay away to negligible levels. It is thought that the universe was created from a huge mass of subatomic particles and energy—the Big Bang Theory. 

Of the elements and their isotopes that constitute our planet, the vast majority are quite stable, the result of billions of years of nuclear decay. The amount of radiation given off from natural radioactive minerals in the earth's crust is a major constituent of background radiation. For the most part, it is quite low, due to the long time required for the remaining radioisotopes to decay. In atomic reactions (either natural or forced by man) the decay process is sped up by the effect of neutrons given off in the fission process interacting with more unstable isotopes to cause immediate decay. While this allows the energy of the isotope to be harvested in a conveniently short time, the unstable decay products produced generally have short half-lives, on the order of seconds to centuries, and are very radioactive. As a result of this process, considerable larger quantities of short half-life (high decay rate) isotopes become a part of the world we live in. This is the basis for the controversy and concerns on the subject of nuclear power generation, waste disposal and nuclear weapons. 

Interaction of Radiation with Matter

The particles and photons that result from nuclear decay carry most of the energy released from the original unstable nucleus. The value of this energy is expressed in electron Volts, or eV. The energy of beta and alpha rays is invested in the particles' speed. A typical beta particle from Cesium-137 has an energy of about 500,000 eV, and a speed that approaches that of light. Beta energies can cover a wide range, and many radioisotopes are known to emit betas at energies in excess of 10 million eV. The penetration range of typical beta particles is only a few millimeters in human skin. 

Alpha particles have even shorter penetration ranges than beta particles. Typical alpha energies are on the order of 5 million eV, with ranges so short that they are extremely difficult to measure. Alphas are stopped by a thin sheet of paper, and in air only travel a few inches at most before coming to a stop. Therefore, alpha particles cannot be detected without being in close contact with the source, and even then only the alphas coming from the surface of the source can be detected. Alphas generated within the source are absorbed before reaching the surface. Due to short range, alpha particles are not a serious health hazard unless they are emitted from within the body when their high energy, in close contact with sensitive living tissue, is an extreme hazard. Fortunately, almost all alpha-emitting substances also emit gamma rays, allowing for their detection. 

Neutrons, having no net charge, do not interact with matter as easily as other particles, and can drift through great thickness of material without incident. A free neutron, drifting through space, will decay in an average of 11.7 minutes, yielding a proton and an electron (beta ray). The neutron can also combine with the nucleus of an atom, if its path carries it close enough. When a neutron is absorbed into a nucleus, it is saved from its ultimate fate (decay), but may render the nucleus unstable. This absorption process is used in medicine and industry to create radioactive elements from non-radioactive ones. Detecting neutrons is specialized and beyond the scope of typical Geiger counters, but most possible neutron sources also emit gamma and beta radiation, affording detection of the source. 

The highly energetic X-ray and gamma rays lose their energy as they penetrate matter. X-rays have an energy of up to about 200,000 eV, compared to gamma radiation which can be as energetic as several million eV. One million eV gamma radiation can penetrate an inch of steel. Gamma and X-ray radiation are by far the most penetrating of all common types, and are only effectively absorbed by large amounts of heavy, dense material of high atomic number, such as lead. 

International System of Units

In 1975, the International Commission of Radiation Units and Measurements (ICRU) recommended that the becquerel be adopted as the standard unit for the measurement of radioactivity and be included in the International System of Units (SI).  Though many countries have already adopted the becquerel as the standard unit of radioactivity measurement, ICN has chosen to indicate size quantities with curies (Ci) as the measurement standard while indication specific activities in both curies and becquerels.  Furthermore, simple-to-use conversion charts have been provided. 
 
Conversion Table

Curies to Becquerels mCi to kBq mCi to MBq Ci to GBq		Curies to Becquerels mCi to MBq mCi to GBq Ci to TBq
1	37	35	1.29
2	74	40	1.48
3	111	45	1.66
4	148	50	1.85
5	185	55	2.03
6	222	60	2.22
7	259	65	2.4
8	296	70	2.59
9	333	75	2.77
10	370	80	2.96
15	555	85	3.14
20	740	90	3.33
25	925	95	3.51
30	1110	100	3.70

 Converting SI units/non-SI units

From	To	Multiply By
becquerel (Bq)	curie	2.7x10-11
curie (Ci)	becquerel	3.7x1010
gray (Gy)	rad	100
rad (rad)	gray	0.01
sievert (Sv)	rem	100
rem (rem)	sievert	0.010