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Information Directory

Reference Directory

Instrument Characteristics

Air Proportional Counter

  • Types of Radiation Measured – alpha
  • Typical Measurement Units – Counts per minute (cpm)
  • Primary Use –  Alpha contamination surveys
  • Typical Background Response – Near zero (0)
  • Advantages – Detects alphas only; Light weight; No special counting gas required
  • Disadvantages – Humidity sensitive; High maintenance

Gas-flow Proportional Counter

  • Types of Radiation Measured – alpha, beta
  • Typical Measurement Units – Counts per minute (cpm)
  • Primary Use – Alpha/beta contamination surveys
  • Typical Background Response – Near zero (0)
  • Advantages – Detects primarily alphas, but has some beta response; Counting gas is readily-obtainable; Insensitive to Humidity; Can be used in the presence of high ambient gamma fields; Alpha and beta counting efficiency is about 40% (2-pi).
  • Disadvantages – Relatively heavy; Counting gas is flammable; Exhibits fair isotopic selectivity for alphas, none for betas

Geiger-Mueller (GM) – End Window and Pancake

  • Types of Radiation Measured – alpha, beta, gamma, x-ray
  • Typical Measurement Units – Counts per minute (cpm)
  • Primary Use – Personnel surveys; contamination surveys
  • Typical Background Response – Less than 100 cpm
  • Advantages – Inexpensive; Simple and reliable; Rapid response time; Sensitive to most contamination types
  • Disadvantages – Large dead time (loss of counts at high count rates; Will not detect very low energy betas and alphas ( <70 keV, < 4 MeV); Energy dependent

Geiger-Mueller (GM) – Side Wall

  • Types of Radiation Measured – beta, gamma, x-ray
  • Typical Measurement Units – Counts per minute (cpm)
  • Primary Use – Personnel surveys, area surveys, equipment surveys, contamination surveys
  • Typical Background Response – Less than 100 cpm
  • Advantages – Inexpensive; Simple, reliable; Rapid response; Easily adjusted to respond to only gammas, x-rays
  • Disadvantages – Large dead time; Will not detect alphas at all or low energy betas (<200 kev); Energy dependent

Ionization Chamber

  • Types of Radiation Measured – gamma, x-ray ( if special window is provided)
  • Typical Measurement Units – mR/hr, R/hr, mrad/hr if special calibration applied
  • Primary Use – Assessment of radiological conditions near gamma and x-ray sources
  • Typical Background Response – 10 to 100 microR per hour
  • Advantages – Directly measures exposure rate (mR/hr); Little-to-no dead time; Can measure very high exposure rates
  • Disadvantages: Slow response time; Sensitive to temperature, pressure and humidity; Under typical configurations, insensitive to particulate radiation.

Neutron REM Meters

  • Types of Radiation Measured – neutron
  • Typical Measurement Units – mrem/hr
  • Typical Background Response – A few mrem/hr (from ambient gamma radiation)
  • Advantages – Can provide tissue-equivalent dose information; Using moderators of various diameters, can provide spectral information; Certain detectors (BF3 proportional counter) are insensitive to gamma to 500 R/hr
  • Disadvantages – Bulky; Fragile; Energy Dependent; Spectral unfolding can be difficult

Plastic Scintillator

  • Types of Radiation Measured – alpha, beta, gamma, neutron
  • Typical Measurement Units – Counts per minute (cpm)
  • Primary Use – Contamination surveys
  • Typical Background Response – 1 cpm for betas and gammas
  • Advantages – Detector can be shaped to suit purpose; Beta efficiency is about 40%.
  • Disadvantages – Gamma sensitivity is generally poor; Poor isotopic selectivity; Relatively expensive.

Sodium Iodide (NaI) Scintillator

  • Types of Radiation Measured – gamma, x-ray
  • Typical Measurement Units – Counts per minute (cpm) or microR per hour if special calibration applied
  • Primary Use – Surveys of low-level ambient gamma radiation environments
  • Typical Background Response – Up to several thousand cpm
  • Advantages – Very sensitive to the presence of gamma radiation; Rapid response time
  • Disadvantages – Detects only gamma’s, x-rays; Relatively expensive; High background; Fragile

Zinc (ZnS) Sulfide Scintillator

  • Types of Radiation Measured – alpha
  • Typical Measurement Units – Counts per minute (cpm)
  • Primary Use – Alpha contamination surveys
  • Typical Background Response – Near zero (0)
  • Advantages – Detects alphas in the presence of penetrating radiations; Light weight; Alpha efficiency is about 40% (2-pi).
  • Disadvantages – Entrance window is fragile; Detector is sensitive to visible light; Fragile in that misalignment of prisms renders device inoperable; No isotopic selectivity.

For additional information about radiation detectors and their use, see Radioactivity Basics