Radiation Instrumentation

Liquid Scintillation Counter (LSC)
Geiger-Mueller (GM) “Pancake” Counter (GM)
Sodium Iodide (NaI(Tl)) Scintillator
Ion Chambers (IC)

Detection Method
Type of Radiation Geiger-Muller Sodium Iodide Liquid Scintillation Counter Ion Chamber
Low Energy Beta ✓
Mid to High Energy Beta ✓ ✓ ✓
Low Energy Gamma/X-ray ✓ ✓
Mid to High Energy Gamma/X-ray ✓ ✓
Alpha ✓ ✓ ✓

Liquid Scintillation Counter (LSC)

The LSC: (a) has the highest sensitivity and can detect Tritium (H-3), other low-energy beta emitters, and other RAM; (b) uses chemicals to convert ionizing radiation into measurable light pulses, and (c) is the slowest method of detection since it take minutes to hours to process results. The LSC is not portable.

It detects radiation from wipe samples dissolved in a solvent mixed with a phosphor. The content of the solution, or “cocktail,” is 60-99% solvent, with the phosphor only making up a small percentage. Any radioactive material on the wipe dissolves in the solution, and the ionizing radiation it emits interacts with the solvent and phosphor molecules. These molecules convert that radiation energy into light pulses, which are counted to give a measurement of radioactivity, shown in counts per minute (CPM) and disintegrations per minute (DPM).

The diagram illustrates a radioactive molecule emitting a beta (β) particle, which immediately encounters a solvent molecule and energizes it (*). That energy flows to the fluor (phosphor) molecule, which converts it to light (hν). This light is detected by the photomultiplier tube, which converts it to an electronic signal and amplifies it. The intensity of the light pulse corresponds to the energy of the radiation emitted, and the number of pulses per second corresponds to the radioactivity, or emissions (decays) per second.

Geiger-Mueller (GM) “Pancake” Counter

GM counters: (a) are the instrument of choice for contamination surveys, (b) are portable, durable, and relatively inexpensive, and (c) cannot detect low-energy radiation. Features of GM counters include:

  • Wide range of energy sensitivity – detection of alpha, beta, gamma, and X-rays in the range of 50 – 3000 keV.

  • Fast response time and audible indicators – ideal for quick surveys for contamination or radiation leakage through shielding.

  • Little to no sensitivity to low-energy radiation.

A Geiger-Mueller (GM) counter, also called a “pancake probe” due to the flat shape of the detector, uses gas ionization to detect ionizing radiation. The flat probe is attached by wire to a general survey meter (metal box with display; Ludlum models). This configuration allows use of other types of probes (e.g., Sodium Iodide (NaI(Tl)) Scintillator; see next section) with the same meter.

A tube within the flat detector is filled with gas and has electrodes on both sides. A high voltage is applied across the electrodes, usually around 1,500 volts. This voltage is higher than that used in ionization chamber detectors (e.g., Fluke 451B). Because of this, the GM counter is less accurate, but more sensitive.

When ionizing radiation passes through the gas chamber, it causes gas molecules to ionize and travel toward the charged electrodes, sending a pulse of current. This pulse is measured by the electronics inside the meter. The number of pulses per second indicates the intensity of radiation present, shown in counts per minute (CPM).

Sodium Iodide (NaI(Tl)) Scintillator

A sodium iodide NaI(Tl) scintillator is another common type of radiation detector. The sodium iodide crystal (NaI) is interspersed with thallium (Tl), the added activator.

The sodium iodide/thallium crystal (called a “scintillator”) becomes energized when struck by gamma or X-rays and emits visible light. This light strikes a “photocathode” (a thin foil) in the device; the photocathode, in turn, ejects an electron. As the electron travels down the photomultiplier tube, it is greatly amplified eventually reaching the anode and sending a signal to the meter. These signals are displayed as CPM.

These detectors are preferred for detecting low-energy gamma or X-rays, but cannot detect alpha or beta radiation because the charged particles do not penetrate the crystal. They are faster and more sensitive than GM counters, however, they are less durable.

Advanced scintillators with appropriate meters can be used to identify the energies of gamma rays. These are called “multi-channel analyzers” and are commonly used to identify the isotopes in unknown radioactive contamination.

Ion Chambers (IC)

Ion chambers are similar to GM counters in that they use a gas-filled chamber with a voltage applied across it. The voltage is lower than in GM counters, around 105 V. This allows for proportional counting of the energy received from radiation as opposed to simply counting the number of pulses. This enables the instrument to accurately measure exposure and exposure rate in milliroentgens (mR) or mR/hr, respectively.

A common IC used at USC is the Fluke Model 4518 Survey Meter. Its features include:

  • Sensitive to alpha radiation above 7.5 MeV, beta radiation above 100 keV, and gamma radiation above 7 keV. Regardless of the source, it can accurately measure the exposure.
  • More expensive than GM counters and somewhat more fragile.
  • The IC takes a few minutes to warm up before giving accurate readings. Be aware that it may show large exposure rates when first turned on whether radiation is present or not.
  • Slower response time than GM counters, ICs generally are poor for detecting contamination. They are best used for measuring the exposure rate once contamination is identified with a GM or NaI(Tl) detector.

References

LSC

GM

Sodium Iodide

Ion Chamber