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, go to Lesson 5 in the "Radioactivity Basics" section (red button on the left).