National Physical Laboratory

What is the right instrument?

3. For contamination monitoring?

cautionRadiation
For each of the three types of potential contaminant, α, β and X or γ photon, there is a variety of types of instrument available.


Potential alpha contamination

  • Alpha emissions are characterised by their extremely short range (a few mm in air) and the very high rate of energy deposition.
  • Monitors require very thin windows (approximately 1 mg cm-2) to allow the particle to pass into the detecting volume but then have very high rates of energy deposition within the detector which can be used to give good discrimination against signals produced by β and X or γ radiation.
  • All monitors should have a very low background, typically a few counts per minute. If it is higher than normal, the probe is probably contaminated and should be cleaned.
alphaDetector

Typical detectors include:

  • zinc sulphide scintillation detectors 
  • dual phosphor detectors
  • thin-window proportional counters
  • pancake style GM detectors

 

Light leaks in scintillation detectors will generally show up as an increase in background. Scintillation detectors should have a light leak detection mechanism to avoid any failure to danger.

Alpha detector


Potential beta contamination

  • Beta contaminants found in practice range from 3H (Emax = 0.018 MeV) to 106Rh (Emax = 3.6  MeV). 
  • In the majority of circumstances, the lowest energies cannot be directly detected on a surface. 14C (Emax = 0.156 MeV) is the lowest energy b emitter that is normally considered to be detectable but even this is challenging.

Typical detectors include:

  • scintillation detectors
  • refillable or sealed proportional counters
  • thin-end-window or thin-walled GM detectors

Light leaks in scintillation detectors will generally show up as an increase in background. Scintillation detectors should have a light leak detection mechanism to avoid any failure to danger.

NOTE

Some instruments discriminate between alpha and beta contamination  -Dual Alpha and Beta contamination probes

  • These instruments display alpha and beta count rates in separate channels. 
  • While the contribution of beta radiation to the alpha channel should be negligible, the alpha activity will normally contribute significantly to the beta channel count rate. 
  • Correct setting of the operating point is even more important for dual probes than for separate alpha and beta probes.

Potential X, γ contamination

  • Many radionuclides emit γ radiation. This emission is normally preceded or accompanied by the emission of a β particle or other radiation.
  • Contamination is normally monitored using the β emissions rather than the γ radiation because β monitors have lower background count rates, higher detection efficiencies and a response that is far less influenced by activity not directly under the detector. The only circumstance when the γ emissions are normally used for the monitoring is when there is contamination in bulk. This situation will not be considered here.

Examples of this decay pattern are:

1.      137Cs which decays by emission of a beta particle to 137mBa, a nuclide with a 156 s half-life that in turn decays with the generation of a 0.662 MeV g-ray.
2.      60Co which emits both a relatively low energy b (0.31 MeV) and two energetic g- rays (1.17 MeV and 1.33 MeV).
A large number of radionuclides decay by electron capture and  emit low to medium energy X-rays. These nuclides, including 55Fe, 57Co and 125I, find frequent use in research and medical applications.

There are two popular types of detector for monitoring contamination by electron capture nuclides:

  • thin-windowed sodium iodide scintillation detectors
  • titanium windowed xenon filled proportional counters.

Both types of detector have a good X and γ detection efficiency but the lower energy threshold varies with window type and should be confirmed. The X-rays are only weakly attenuated by air, which means that monitoring can take place with the probe at a distance from the surface of interest.

The main problem tends to be the presence of other sources in the area, producing a high local background making the detection of contamination more difficult. One option is to use a detector, which has a lead sleeve surrounding the scintillator (sometimes described as collimated detectors). Their use will reduce the background where the source of radiation is well off the axis of the detector, e.g. a source safe in a distant corner of the bench being monitored. The disadvantage of collimated detectors is that they are heavier.

Detector characteristics

 

X,γ,β Dose equivalent rate measurement

Neutron dose equivalent rate monitoring

Alpha contamination monitoring

Beta contamination monitoring

Detector types suitable for different radiations


X,γ,β Dose equivalent rate measurement

DETECTOR  TYPE

STRENGTHS

THE Ionisation Chamber
  • can be made to have a very good X, γ energy and polar response and acceptable β characteristics
  • no problems with pulsed fields
  • generally good dynamic range of dose rates, typically 2 mSv h-1 to 10 mSv h-1 but on other types 2 mSv h-1 to 10 Sv h-1

Dose Rate Measuring Proportional Counter
  • good X, γ energy response down to ~ 30 keV
  • useful beta response at higher energies
  • generally satisfactory with pulsed fields
  • good sensitivity for detector size
  • wide dynamic range of usable dose rates. Low dose rates often use high gain (high detector polarising voltage) and high dose rates a lower voltage and hence lower gain
  • sometimes an audio output

Steel Walled Energy Compensated GM

 
  • very easy to process signal
  • much more sensitive than an ion chamber. As an example, a GM with a volume of 10 cm3 will produce as stable an indication on the meter as an ion chamber of 300 cm3
  • consistent. The operating voltage is fixed and the sensitivity on the larger ones varies little
  • stable and long lived, if undamaged
  • low cost
  • audio output

Thin End Window, Energy Compensated GM
  • very good X, γ energy response from 10 or 15 keV upwards to 1.25 MeV
  • good polar response

End Window GM Detectors
  • respond to X, γ radiations from 5 keV upwards and to all β radiation which contributes to ambient or directional dose equivalent rate
  • good polar response (pancake types)

Plastic Scintillator Based Instruments

 
  • good X, γ energy and polar response for ambient dose equivalent rate down to 20 keV for instruments with smaller scintillators and thin cans
  • high sensitivity. A 50 mm x 50 mm plastic scintillator will operate reasonably well at background levels
  • good dynamic range. In the same way as the proportional counter they can be operated at high voltage (hence high gain) at low dose rates and lower voltage (hence low gain) at high rates
  • easy to produce a logarithmic dose rate response

Sodium Iodide Based Scintillation Detectors
  • very high sensitivity
  • nuclide identification
  • generally an audio output

 

WEAKNESSES
  • very low signal level at normal radiation protection dose rates leading to poor levels of statistical fluctuation or slow response times
  • generally unusable below 2 mSv h-1
  • susceptible to temperature and humidity problems
  • requires good maintenance, particularly regular drying of desiccant
  • often use unusual polarising batteries
  • generally no audio output
  • relatively vulnerable detector, for the β versions
  • uses a very high polarising voltage
  • expensive
  • susceptible to high voltage variation
  • no useful β response and an X, γ response that falls rapidly below ~ 50 keV
  • problems in pulsed fields. Untrustworthy when the count rate from the detector exceeds about 35% of the pulse rate from a machine producing narrow (ms) pulses. Ultimately, in the main beam, perhaps at Sv h-1, the detector will respond to each pulse, not to the dose rate. This will lead to a potentially serious under-response
  • instruments where the filter can be removed so that the detector can be used as a conventional end window detector are susceptible to damage
  • if used with the window unprotected, i.e. for β and very low energy X, γ measurements, they are very vulnerable to damage. Damage to the window is generally fatal. They cannot be repaired. They must be protected with a fine etched metal or plastic grill
  • poor energy response for X, γ radiation
  • large detector (scintillator and photomultiplier tube)
  • expensive
  • generally no audio output
  • very expensive
  • limited dynamic range


Neutron dose equivalent rate monitoring

DETECTOR TYPE

STRENGTHS

BF3 Proportional Counter, Spherical Moderator
  • near isotropic response
  • good γ rejection

3He Proportional Counter, Spherical Moderator
  • near isotropic response

BF3 Proportional Counter, Cylindrical Moderator
  • good γ rejection

LiI(Eu) SCINTILLATOR, SPHERICAL MODERATOR
  • reasonably light, for a neutron monitor

 

 

WEAKNESSES
  • BF3 is poisonous
  • 3He is expensive
  • non isotropic response
  • BF3 is poisonous
  • poor sensitivity, typically 0.2 s-1mSv-1h
  • an energy response inferior to the cylindrical form
  • poor γ rejection
  • variable operating voltage

Alpha contamination monitoring

DETECTOR TYPE

STRENGTHS

Solid State Detectors
  • very good detection efficiency
  • very lightweight and compact

Zinc Sulphide Scintillation Detectors
  • good detection efficiency. The majority of a particles that penetrate the window with significant energy will be counted
  • available in a wide range of sizes
  • reasonable β and X and γ rejection although ultimately either false counts will be recorded at high dose rates or the detector will fail to danger
  • lightweight. Most instruments use separate probes
  • low intrinsic background
  • easy setting up procedure (but see below)

Dual Phosphor Scintillation Probes

(Zinc SULPHIDE ON PLASTIC SCINTILLATOR)
  • good a detection efficiency, as for standard a pulses
  • useful for mixed a and high to medium energy b contamination
  • lightweight
  • easy window repair

Thin Windowed, Gas Refillable Proportional Counters

 
  • very good detection efficiency. Virtually any a particle passing through the window with an energy in excess of 0.5 MeV will be counted
  • available in very large sizes, if required
  • a very good b detection efficiency, operating well down to the energy of 14C, 0.156 MeV
  • easy window repair
  • consistent operating potential
  • not influenced by magnetic fields

 

WEAKNESSES
  • extremely susceptible to electromagnetic interference
  • vulnerable detector. Unlike the scintillator and photomultiplier combination, the delicate, expensive part is just behind the window
  • sensitive to high magnetic fields, unless filled with a mu metal screen
  • the uniformity of the larger detectors can be poor, with a low response to activity in the detector corners
  • very variable optimium operating voltage, normally between 700 and 1200 V
  • more complicated setting up procedure, which is also more demanding on the ratemeter
  • sensitive to high magnetic fields, unless filled with a mu metal screen
  • very variable operating voltage
  • require regular refreshing with counting gas
  • they are not suitable for intermittent use
  • operation at very high voltages (1.5 ® 2 kV) may cause problems in high humidity


Beta contamination monitoring

DETECTOR TYPE

STRENGTHS

Beta Scintillation Detectors
  • available in a wide range of sizes
  • can cover a wide range of energies
  • inefficient response to lower energy X, γ radiation, helping to minimise background
  • window easily replaced
  • lightweight
  • easy setting up procedure

Thin Windowed, Gas Refillable Proportional Counters
  • very good detection efficiency
  • available in very large sizes, if required
  • a very good b detection efficiency, operating well down to the energy of C-14, 0.156 MeV
  • easy window repair
  • consistent operating potential
  • not influenced by magnetic fields
  • good a rejection

Titanium Windowed, Xenon Filled Sealed Proportional Counters
  • useful for β and low energy X and γ radiation
  • relatively tough window
  • lightweight
  • no gas filling required
  • consistent operating potential and radiation characteristics

Thin End Window Geiger Muller Detectors
  • large, easily processed pulse
  • very simple setting up procedure
  • consistent operating voltage and radiation characteristics
  • lowest cost overall option in most circumstances
  • light and compact

Thin walled Geiger Muller Detectors
  • more robust than the thin window variety
  • larger useful area than the thin window variety
  • very simple setting up procedure
  • consistent operating voltage and radiation characteristics
  • low cost
  • light

 

WEAKNESSES
  • susceptible to magnetic interference. This may be a problem
  • very variable operating potential within any one type
  • uniformity of larger detectors can be poor
  • no alpha discrimination unless in 'dual phosphor probe' form
  • require regular refreshing with counting gas
  • they are not suitable for intermittent use
  • operate at very high voltages (1.5 ® 2 kV)
  • window not easily replaced. Repair costs of the order of £500 (2000)
  • higher background than all other types per unit area
  • windows are extremely vulnerable and cannot be repaired
  • not available in large sizes
  • relatively high minimum useful energy
  • unrepairable


X, γ Contamination

DETECTOR TYPE

STRENGTHS

Thin Windowed, Thin Sodium Iodide Scintillator
  • the thin crystal is a very efficient X, γ photon detector. For the 3 mm thickness the detection probability is greater than 0.5 for normal incident radiation up to 120 keV
  • a typical aluminium window, 14 mg cm-2 thick has a transmission of at least 0.8 for normal incident X, γ photon radiation down to 10 keV. For a typical beryllium window, 46 mg cm-2 the transmission at normal incidence is at least 0.8 down to 5 keV. The combination of the scintillator and window thus offers a very efficient detector over a wide energy range

Xenon Filled, Titanium Windowed Proportional Counters
  • useful for β and low energy X and γ radiation
  • relatively tough window
  • lightweight
  • no gas filling required
  • consistent operating potential and radiation characteristics

 

WEAKNESSES
  • the scintillator is very brittle and easily crazes with mechanical shock
  • window damage, which is not carefully repaired, will lead to a gradual deterioration of the scintillator, resulting in an increase in the energy threshold
  • quite expensive
  • window not easily replaced. Repair costs of the order of £500 (2002 prices)
  • higher background than all other types per unit area


Detector types suitable for different radiations

Radiation type

Energy range

Detector type

Typical Characteristics

Low energy Gamma/X-ray

10 keV Þ

Ionisation chamber

Large volume detector required at protection level dose rates. Provides H*(10) answers. Requires careful use, expensive. Poor response at low dose rates. Flat energy response. Some instruments will have poor polar response at low energies.

 

 

 

Thin end window GM tubes, energy compensated

Inexpensive. Energy response as good as ion chambers, good response at low dose rates.

 

 

 

Plastic scintillator

Good energy response down to 20 keV,  high sensitivity therefore useful at low dose rates.

 

 

 

Thin sodium iodide detectors

Very high sensitivities at low energies. Abysmal energy response. No use for dose rate assessment. Good for low energy photon sources and 'Search & locate'.

 

 

 

Proportional counter

Uses gas amplification to produce a good signal at low dose rates.

Medium energy Gamma/X-ray

60 keV Þ

Compensated GM tube

Cheap and rugged. Operates well at low dose rates. Provides H*(10) answers. Energy response not flat but generally within ± 30 % over range.

 

 

 

Ionisation chamber

Large volume detector required at protection level dose rates. Provides H*(10) answers. Requires careful use, expensive. Poor response at low dose rates, flat energy response. Some instruments will have poor polar response at low energies.

 

 

 

Proportional counter

Uses gas amplification to produce a good signal at low dose rates. Expensive.

 

 

 

Large volume Sodium Iodide detectors

Very high sensitivity. Limited range of dose rate levels (< 50 mSv h-1). Good energy discrimination and therefore nuclide identification. Expensive. Requires careful use.

Beta dose rate

 

Ionisation chamber

Large volume detector required at protection level dose rates. May provide H¢(0.07) answers. Requires careful use, expensive. Poor response at low dose rates. Instruments will have poor polar response.

 

 

Thin end window GM tube

Sensitive, reasonable b energy response but poor X, γ response.

 

Radiation type

Energy range

Detector type

Comments

Alpha contamination

4 MeV Þ

Solid state detectors

Good detection efficiency. Susceptible to RF and tend to be microphonic. Lightweight units. Expensive. Fragile detector.

 

 

Zinc sulphide scintillator

Commonly available, good sensitivity, fragile foil. Can be g sensitive. Many different sizes.

 

 

Thin end window GM

Fragile. Background countrate generally too high for most applications. No discrimination against other radiations. Small pancake GMs reasonably cheap.

 

 

Large area gas re-fillable proportional counters

Tend to be expensive. Good detection efficiency. Require very high voltage. Discrimination against b radiation possible. Requires separate gas supply.

Beta contamination

0.15 MeV Þ

Anthracene scintillator

Commonly available. Good sensitivity. Fragile foil. Many different sizes.

 

 

Plastic scintillator

Readily available. Sensitivity not as good as anthracene. Inferior low energy response. Fragile foil. Many different sizes.

 

 

Thin end window GM

Fragile. No discrimination against other radiations. Small pancake GMs reasonably cheap.

 

 

Large area gas re-fillable proportional counters

Tend to be expensive. Good detection efficiency. Require very high voltage.

 

 

Large area sealed proportional counters

Expensive. Good detection efficiency but not at low b energies due to rugged titanium window. Require very high voltage.

 

 

Thin walled GM tubes

No good for bs with an Emax<0.5 MeV. Glass tubes can be very expensive.

Neutrons

Thermal Þ

BF3 proportional counter Spherical moderator

 

Poor sensitivity, typically 0.2 to 0.7 s-1mSv h-1 for 241Am-Be.

 

 

3He proportional counter Spherical moderator

Poor sensitivity, typically 0.2 to 0.7 s-1mSv h-1 for 241Am-Be.

 

 

BF3 proportional counter cylindrical moderator

Poor sensitivity, typically 0.2 to 0.7 s-1mSv h-1 for 241Am-Be.

 

 

LiI(Eu) proportional counter Spherical moderator

Poor sensitivity, typically 0.2 s-1mSv h-1.

 

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