At the basic physics level, we are often most interested in the activity of a source, which is the number of decay events per unit time. The SI unit of activity is the becquerel (1 Bq = 1 nuclear decay per second), but it is still common to see an older historical unit called the curie (1 Ci = 3.7 x 1010 Bq) which was originally defined as the the number of decay events occurring in one gram of radium. Note that activity is a measure of the number of nuclear decay events, not the number of emitted radioactive particles; the number of emitted particles can vary from decay event to decay event and typically is greater than one emission per nuclear decay.

Activity is property of the source, but intensity passing through a detector (i.e., the number of radioactive particles per unit time) is often more useful than the source activity. Intensity is typically reported in units of counts per second (cps) or counts per minute (cpm).

The simplest device for measuring particle intensity is a Geiger-Mueller (GM) counter , also sometimes simply called a lab survey meter .  A GM counter is sensitive to alphas, betas, x-rays, and gammas, but it cannot differentiate between particle types or energies. A GM counter also has a range of efficiencies depending on particle type and energy. Some other lab survey meters (like the one shown in Fig. 1) use small NaI(Tl) scintillator crystals for increased gamma efficiency.

Survey meters are useful for detecting whether or not radiation is present and for comparing relative strengths of sources, but more sophisticated particle detectors are required to measure absolute activity.

Figure 1: A survey meter equipped with a NaI(Tl) probe for measuring gamma intensity.

For a given source of activity A which emits i different particles each with branching ratio (decay probability) Pi, the total measured intensity I is given by

where$\Omega$  is the solid angle subtended by the detector and $\epsilon_i$  is the detector efficiency for the _i_th particle.

Different types of radiation have different interaction mechanisms and different energies. They will therefore have different impacts on materials.

An appropriate quantity which takes these differences into account is the absorbed dose, which is defined as the total amount of energy deposited per unit mass in a particular material. This quantity varies with particle type and energy, and also varies with the type and density of the material. The SI unit of absorbed dose is the gray (Gy = 1 J/kg), but the older historical unit is the rad (1 rad = 0.01 Gy). Both these quantities correspond to total dose; it is common to see dose rates in units of Gy/s or rad/s.

One last complicating factor remains, though. Total energy deposited by different types of radiation are not biologically equal. It therefore is useful when talking about the health impacts of radiation to define equivalent dose as the absorbed dose multiplied by a weight factor which depends on the type of the radioactive particle. This weighting accounts for biological differences in the effects of ionization from the radioactive particles. For example, energy liberated through damage by an alpha particle is considered to be 20 times more damaging than the equivalent amount of energy liberated by a gamma, and therefore the equivalent dose for alphas is 20 times the absorbed dose. Weight factors are shown in Table 1. The SI unit of equivalent dose is the sievert (1 Sv = 1 J/kg) and the older historical unit is the roentgen equivalent man or rem (1 rem = 0.01 Sv).

Table 1: Equivalent dose weights by radioactive particle type (Source: Wikipedia)

Calibrated detectors capable of measuring equivalent dose rate exist. For example, ion chamber survey meters are good for monitoring gammas and high energy beta particles, whereas He-3 gas-filled chambers can be used to detect neutrons. 

Because humans are exposed to many naturally occurring sources of radiation (e.g. radon gas in the soil, cosmic rays) as well as common medical and industrial sources (e.g dental x-rays), the average person has a background eqivalent dose rate of about 350 mrem/year. Guidelines on building materials and devices recommend limiting additional general public exposure to no more than 100 mrem/year. In addition, high-rate radiation exposure is to be limited to no more than 2 mR in one hour. (The body can recover from long-term low-level exposure better than short-term high-level exposure.)

For those who work regularly with radioactive material and who receive appropriate training, recommended limits are much higher. The annual dose limit for radiation workers is 5 rem/year for the whole body radiation, or 50 rem/year limited to extremities (like arms or legs). Radiation workers who expect to receive more than 500 mrem/year are required to wear a dosimeter badge which monitors their total dose. More information about limits for radiation workers can be found on the OSHA website.

Table 2: Maximum permissible radiation doses

When used safely, the radioactive sources in this course are expected to deliver total doses well below safe limits and well below the limits where a user is required to wear a dosimeter badge.

The whole body effective gamma dose rate at a particular distance from a point source is calculated as

where $\Gamma$  is the so-called gamma dose constant. This constant can be calculated for different sources according to the energy and probability of each decay gamma in the source. Information about calculating this constant (as well as an extensive table of values for common radionuclides) is available in this Oak Ridge National Laboratories report.

Several $\Gamma$ constants for radioactive sources used in our labs are given in Table 3.

Table 3: Sources commonly used in the Physics Instructional Laboratories and their gamma ray constants.

As an example of the typical dose rates you will encounter in this course, consider a 10 $\mu$Ci Na-22 button source. Such a button – when used at a distance of 1 m – provides a dose rate of

Even if you spent all day, every day in front of this source for a year (8760 consecutive hours), you would still barely reach the general public dose limit of 100 mrem.