Radiation Detectors
Survey Meters
Geiger-Mueller (GM) Survey Meters
Low Energy GAMMA (LEG) or Scintillation Survey Meters
Ion Chamber Survey Meters
Liquid Scintillation Counters
Radiation Dosimeters
Film Badges
Thermoluminescent Dosimeter (TLD)
Optically Stimulated Luminescence Dosimeters (OSL)
Personal Dosimetry Program
Radiation Detection and Measurement Techniques
Survey Meters (continued)
Operating Procedures for Radiation Survey Meters
Liquid Scintillation Counters (continued)
Basic Operating Procedures for LSC'S

Because human beings cannot see or feel radiation it is necessary to rely on monitoring and detecting equipment to measure the amount of radioactivity present. A variety of instrumentation is available for that purpose. Radiation is detected using special detectors which measure the amount or number of ionizations or excitation events that occur within the device. The radiation detection system can be either active or passive, depending upon the device and the mechanism used to determine the number of ionizations. Active devices provide an immediate indication of the amount of radiation or radioactivity present. Passive devices are usually processed at a special processing facility before the amount of radiation exposure can be reported. Portable radiation survey meters and laboratory counting equipment are active devices, while radiation dosimeters used in determining individual exposure to radiation and radon detectors are passive devices. At the end of this chapter, Table 6, lists expected efficiencies for commonly used radioisotopes using different types of survey equipment.

Figure 7 illustrates the basic principle used by portable instruments in the detection and measurement of ionizing radiation. The detector tube (i.e. Geiger counter) is simply a gas filled, cylindrical tube with a long central wire that has a 900-volt positive charge applied to it and is then connected, through a meter, to the walls of the tube. Radiation enters the tube and produces ion pairs in the gas. The electron part of the ion pair is attracted to the positively charged central wire where it enters the electric circuit. The meter then shows this flow of electrons (i.e. the number of ionizing events) in counts per minute (cpm).

The only prerequisite for the detection of radiation with a survey meter is that the radiation must have sufficient energy to penetrate the walls of the detector tube and create ionizations in the gas. Particulate (alpha and beta) radiations have a limited range in solid materials so, radiation detectors designed for these radiations must be constructed of thin walls that allow the radiation to penetrate. The most common types of portable radiation survey meters used in research labs are the thin window Geiger-Mueller (GM), Low energy Gamma (LEG), and Ion Chamber survey meters.

GM survey meters are radiation detection devices used to detect radiation or to monitor for radioactive contamination. GM detectors usually have "window" either at the end or on the side of the detector to allow alpha or beta particles to enter the detector. These detectors may have a variety of window thicknesses but, if the radiation cannot penetrate the "window" it cannot be detected. Depending upon the "window" thickness, GM systems can detect x-ray, gamma, alpha, and/or beta radiation. Common radioactive materials that emit these types of radiation (e.g. 22Na, 32P, 35S, 51Cr, 137Cs) can usually be detected using GM survey meters. Because GM detectors are more sensitive to x-rays, gamma rays, and high energy beta particles and less sensitive to low energy beta and alpha particles, they are usually not used to detect alpha radiation or very low energy beta radiation. Thus, GM survey meters are usually not useful for monitoring 3H or 63Ni, nor are they sensitive enough to detect small amounts (< 1 µCi) of radionuclides that emit low energy beta or gamma radiation such as 14C or 125I. GM meter readings are usually expressed in counts per minute (cpm) for particle radiation or milliroentgen per hour (mR/hr) for X- or gamma rays.

LEG survey meters are radiation detection systems used to monitor radionuclides that emit low energy gamma radiation (e.g., 51Cr, 125I). They can not detect alpha particles nor low energy beta particles but they can detect radionuclides that emit high energy gamma (22Na) and/or high energy beta (32P) radiation. The meter is usually read in counts per minute. When measuring 125I with a LEG survey meter, a reading of approximately 1,000,000 cpm corresponds to a gamma exposure rate of about 1 mR/hr.

Ion chamber survey meters are radiation detection devices designed to collect all of the ion pairs produced in the detector tube and then measure the current flow. These meters are primarily used to measure X- and ?-ray exposure in air and the readings are usually expressed as milliroentgen per hour (mR/hr) or roentgen per hour. Ion chambers are most often used for measuring high levels of X- or gamma radiation exposure and are not often used in research labs.

A scintillator is a material which gives off a photon (flash) of light when struck by radiation. Liquid scintillation counting is a method of assaying a radioactive sample by dissolving it in a mixture of chemicals called scintillation fluid or cocktail. When the radioactive decay energy is absorbed by the solution, the cocktail emits light. The light flashes are converted to electrical signals by a detector called a photomultiplier tube (PMT). These electrical signals are directly related to the absorbed energy allowing the sample to be quantified. Liquid scintillation counters (LSC) are usually used to quantify radioactivity and to measure removable radioactive contamination. They are ideal for counting radionuclides that decay by alpha and beta particle emission (3H, 14C, 32P, 35S) and are also used to measure some low energy gamma emitters (125I) which emit auger electrons as part of their decay.

Because it would be quite impractical to follow each worker around with a survey meter to try to keep track of the radiation exposure fields they enter, Radiation Safety monitors a worker’s external radiation exposure with a personal dosimeter or radiation badge. These devices essentially store-up the radiation energy over the period it is used and is then sent to a vendor to read the exposure and report the results. There are several types of radiation dosimeters commonly used, although the most common are film badges, thermoluminescent dosimeters and the new optically stimulated luminescent dosimeter.

Film is the oldest personal monitoring device and, world-wide is the most common type of personal dosimeter primarily because of its simplicity and east of use. X-rays and gamma rays, along with beta particles, can darken photographic film just as visible light does. This property is the basis for the common film badge. Several different designs are available, but they all have essentially the same components. A piece of film wrapped in paper is inserted into a plastic holder. An "open window" in the plastic allows the passage of low-energy betas which would not penetrate the plastic holder. (Note: tritium betas are so weak they cannot penetrate the paper wrapper of the film, so exposure to tritium cannot be detected on a film badge). Small pieces of aluminum, cadmium or copper, and lead are molded into the plastic holder to act as filters to help differentiate and quantify the energies of radiation the film was exposed to. For example, some gamma rays that penetrate the aluminum may be stopped by the copper, cadmium, or lead. When the film is developed, different areas of blackness appear under the different metals. The film badge vendor can analyze the darkness patterns on the film to determine both the type and amount of radiation which the badge has been exposed to.

In 1953 it was proposed that thermoluminescence be used as a radiation detector. The TLD contains several mineral crystals or "chips" coated with a radiation-sensitive material. There are several different TLD crystals in use depending upon the application but, one commonly used thermoluminescent material is lithium fluoride activated with magnesium and titanium. The chips are enclosed in a plastic case that has an open window area to admit beta particles and three filtered areas to measure the penetration of any gamma doses. When exposed to radiation the TLD absorbs energy from the source which raises the molecular energy of the detector material to a metastable state. The molecules remain in these excited states until, through processing by the vendor, they are heated to a temperature high enough to cause the material to return to its normal state. When this occurs, light is emitted. The amount of light emitted is proportional to the dose received by the TLD. The emitted light is measured with a photomultiplier tube and the dose reading is derived.

A new dosimetry product currently in use at the University is the optically stimulated luminescence (OSL) dosimeter. This dosimeter measures radiation through a thin layer of aluminum oxide. In the OSL dosimeter an aluminum oxide strip is enclosed in a blister pack that has an open window area to admit beta particles and three filtered areas to measure the penetration of any gamma doses. During analysis, the aluminum oxide is stimulated with selected frequencies of laser light, which cause it to become luminescent in proportion to the amount of radiation exposure. The luminescence is measured and a report of exposure results is generated.

Unlike active detectors, the TLD has no readout or display. Radiation workers wear the dosimeter for a given period of time (monthly or quarterly) and return the dosimeter so it can be processed by a vendor. Thus, the worker learns of his/her radiation exposure several weeks after it has occurred.

"Radiation badges" (dosimeters) are generally used to monitor personnel and areas where radiation sources are used. The Nuclear Regulatory Commission (NRC) requires that personnel monitoring be performed if a worker is "likely to receive, in 1 year...doses in excess of 10 percent of the applicable limits." University policy requires personnel to wear radiation badges when using more than 1 mCi of radioactive material which decays by gamma or beta emission with Emax > 200 keV. These dosimeters are used to monitor not just whole body exposure, but also exposure to a worker's hands. Extremity monitors are ring badges with a single chip. Persons working with small amounts of radioactivity or low energy emitters such as 14C, 3H or 63Ni do not need to wear a dosimeter.

If you have been issued a dosimeter to monitor your radiation exposure, you should follow a few simple rules to insure that the dosimeter accurately records your radiation exposure.

• Wear only your assigned dosimeter; never wear another worker's badge.
• Wear your whole body badge between your collar and waist. Wear your ring badge beneath your gloves with the label on the palm side of the hand with the greatest potential for exposure, usually the hand that handles the radiation source.
• Do not store your badge near radiation sources or heat sources.
• If you suspect contamination on your badge, return it immediately to Radiation Safety; you will be given a new, uncontaminated badge.
• Never intentionally expose your badge to any radiation.
• Do not wear your badge when receiving medical radiation exposure (e.g., x-rays, tests, nuclear medicine procedures, mammograms, etc.)
• Return your badge(s) to Radiation Safety at the end of the monitoring period. Snap the “body badge” out of the holder and retain the holder for your replacement dosimeter. Return the complete TLD ring.

The vendor sends all dosimetry reports to the Radiation Safety Program. You will be notified immediately of any overexposure or of levels that warrant investigation. Permanent records of actual doses recorded by the dosimeters assigned to individuals during their affiliation with the University are maintained by this office. You can request your exposure history at any time by calling 229-4275.

Because radioactive material may pose a potential risk to users, all personnel who work with radioactive materials must understand how to use the various types of radiation detection systems to verify that their work place continues to be contamination free. To measure radiation, a workers must first understand how a detector works and then how to use it.

All survey meters have certain controls in common. Figure 9 shows the basic components of a survey meter.

The detector or probe is the device which produces electrical signals when exposed to radiation. It usually has a window through which beta radiation can penetrate its cavity.

The dial or readout is the gauge which indicates the amount of radiation exposure present. It often has two scales, mR/hr and/or CPM. The selector switch is a switch to turn the meter on-off, check the meter batteries, or select a scale multiplier.

The scale multiplier is a number (i.e., 0.1, 1.0, 10, etc.) by which the meter readings must be multiplied to calculate radiation exposure or the number of counts per minute.

The reset button allows the meter reading to be zeroed. When the level of radiation or the number of counts exceeds the highest reading at a particular scale multiplier, switch the scale multiplier to a higher range and push the reset button. This causes the readout needle to reset to zero so the user can accurately determine the count rate.

The response button adjusts the response time of the meter. When this switch is fully clockwise the meter will have a faster response but, the meter readings will be less stable. For response times in the microseconds range, this switch should be turned fully counterclockwise. For routine work set the response button to the slow mode.

The speaker is an audible device connected to the radiation monitor. It may be located outside or inside the meter and may have its own battery. The speaker is in-line with the detector so each count produces an audible click on the speaker.

• Read the instrument's operating manual to gain familiarity with the controls and operating characteristics.
• Check the meter for any physical damage. Look at the calibration certificate and check the date the meter was calibrated. Note that meters are required to be calibrated at least once a year.
• Check the batteries. Turn the selector switch to BATT position. The needle must be within BATT OK range. If not, the batteries are weak and must be replaced. Remember to turn off the instrument when not in use. When storing the meter for extended periods of time remember to remove the batteries and have the instrument recalibrated before resuming use.
• Check the operability of the detector. Radiation Safety places a check source on all meters and records the meter's response with the detector on the source on the calibration sticker. With the meter and speaker turned on, position the selector switch to the appropriate scale, place the detector window over the check source affixed to the side of the meter, and measure the radiation of the source. Compare the response with that recorded on the calibration sticker. This response should be with in + 20% of the indicated response.
• Determine the operating background. With the meter turned on and the selector switch on its lowest scale, point the detector away and/or move away from any radiation fields and measure the background radiation. Note that the meter reading must be multiplied by the selector switch scale (i.e., x 1, x 10, x 100, etc.). The result is the background reading. Normal background readings are about 0.02 mR/hr or 20 to 40 cpm for GM meters, and about 150-200 CPM for LEG meters.
• With the speaker on, point the probe window at the area or equipment you wish to monitor for radiation exposure or radioactive contamination. Unless contamination is expected, place the selector switch on the lowest scale. When surveying or entering contaminated areas with unknown radiation levels, turn the meter on outside the area, place the selector switch on the highest range setting and adjust the switch downward to the appropriate scale. Multiply the meter reading by the selector switch scale - if the needle is on 2 mR/hr, and the selector switch is on the x 10 scale, the radiation exposure is 20 mR/hr.

LSC's come in a variety of shapes and types and manufacturers may use different terminologies, but an overview of terms basic to scintillation counting follows.

Cocktail:

The scintillation fluid. A mixture of chemicals which emits light flashes when it absorbs the energy of radioactive decay.

CPM:

Counts per minute. This is the number of light flashes or counts the LSC registered per minute. The number of decays produced by the radioactivity is usually more than the number of counts registered. Discriminator A circuit which distinguishes signal pulses according to their pulse height. It is used to exclude noise or background radiation counts.

DPM:

Disintegration per minute. This is the number of decays per minute.

Efficiency:

The ratio, CPM/DPM, of measured counts to the number of decays which occurred during a measurement time.

Emulsifier:

A chemical component of the liquid scintillation cocktail that absorbs the UV light emitted by the solvent and emits a flash of blue light.

Fluors:

Chemicals present in the liquid scintillation cocktail that convert the energy of the beta decay to flashes of light.

PMT:

The Photo-Multiplier Tube is the device that detects and measures the blue light flashes from the fluor and converts it into an electrical pulse.

Pulse:

Electrical signal of the PMT; its size is proportional to the radiation energy absorbed by the cocktail.

Quenching:

Anything which interferes with the conversion of decay energy emitted from the sample vial into blue light photons. This usually results in reduction in counting efficiency.

QIP:

The Quenching Index Parameter is a value that indicates the sample's level of quenching. Another parameter that describes the amount of quenching present is the transformed Spectral Index of External Standard (tSIE) or "H" number.

Solvent:

A chemical component of the liquid scintillation cocktail that dissolves the sample, absorbs excitation energy and emits UV light which is absorbed by the fluors.

• Read the instrument operating manual to gain familiarity with the controls and operating characteristics.
• Place your sample into a liquid scintillation vial and add the appropriate amount of liquid scintillation cocktail.
• Prepare at least one background vial. This vial ideally contains a non-radioactive sample similar to your radioactive samples mixed with scintillation cocktail.
• Place your sample vials along with the background vial into an LSC tray and place the tray into the LSC.
• Review the instrument settings and/or counting program to ensure they appropriate for the type of radiation you are counting.
• Begin instrument counting cycle.

Contact the Radiation Safety Program Office for more information on the theory and mechanisms for liquid scintillation counting.

¹Electrons are either Auger or conversion electrons, the efficiency given accounts for abundance
²2GM - GM thin end-window probe; pancake has slightly higher efficiency
LEG - Low Energy Gamma Probe
LSC - Liquid Scintillation Counter
³LSC efficiency will depend on the amount of quenching present in the sample. Values listed are based on 50% quench. GM efficiency is based on the probe's end-cap being"off", efficiency with the cap on is 1/2 these values. GM efficiency is percent of 2p emission rate.