Chris - Radiation Detectors Flashcards
What are the three ways EM radiation produces ion pairs in matter? How do they differ?
- Photoelectric Effect (useful for spectroscopy)
“ absorption of a photon by a bound electron and conversion of the gamma energy in potential and KE of the ejected electron”
• 0-100KeV range
• isolated hits
- Compton Scattering
“Elastic scattering of a gamma ray on a free electron. A fraction of the gamma ray energy is transferred to the Compton electron. The wavelength of the scattered gamma ray is increased.”
•Ce-137 very common in decommissioning 662KeV
Co-60 “” 1.17MeV
- Pair Production
“ if gamma ray energy is»_space; 2m0c2 (electron rest mass 511KeV) a positron-electron pair can be formed in the strong Coulomb field of a nucleus. This pair carries the gamma ray energy minus 2m0c2.
• 10MeV <
• pattern of hit important
Biggest difference between X-ray and Gamma ray photons?
The manner of their production
• Gamma photons come from a radioactive material (can’t necessarily switch off)
• X-ray photons come from tube and can be switched off
What are the most important measurements to make for the nuclear industry?
The energy (principle via calorimetry) and number of events
What is an ion chamber and list it’s pros and cons
Radiation enters air filled chamber producing an ion pair. This pair drift to respective electrodes producing a small current, more photons equals higher current.
Tells you # of events but not source
PROS:
+ve used as a dose rate monitor
+ve measures gamma >12KeV and beta >70KeV
+ve detects neutrons (~8% efficiency)
CONS:
-ve low current requires expensive electronics
-ve very slow response times
-ve susceptibility to humidity and moisture
-ve susceptibility to air pressure changes
-ve contamination by radioactive gases
What is a Geiger Muller Tube and state its pros and cons
Gas ionisation (similar to ion chamber) however, operates at low pressure and high voltages in which the HV causes a cascade effect of events.
PROS:
+ve higher currents means cheaper electronics
+ve easily replaceable tubes
+ve audible and visible outputs
+ve not susceptible to atmospheric changes
+ve sensitivity means is best used as a contamination monitor
CONS:
-ve can’t be used as doserate monitor
-ve requires quenching to stop runaway cascade
-ve fragile end windows
-ve no energy discrimination (can’t detect another event whilst cascade of prior event is occurring)
What is a smear counter and what’s it used for?
Used for measuring contamination on surfaces
Take smear samples in lab or field
Measures smear integration over time, obtains net alpha, beta* and gamma readings**
*differentiates between alpha and beta by using different windows which block the other
**gamma reading important as it removes background signal
How are neutrons detected?
- Cause fission reactions to occur on very small samples of uranium
- Diamond radiation detectors, absorbs neutron and ejects alpha particle which is detectable
**hard to measure directly but can be measured indirectly
What are scintillation detectors and their liquid versions and what is the most common material of choice used?
Utilises scintillator materials which emit light when interacting with radiation. The atoms within the material become excited, emitting a photon of light, the light is then detected by a photo multiple tube (photodiode) which converts the light to an electrical signal which is then analysed.
Efficiency depends on electron density with sodium iodine crystals having the highest density and are the most common
• discriminates between radioisotopes
Liquid scintillators typically used for beta emitters (dissolve sample in liquid and measure light flashes in emitting liquid)
Pros of liquid:
+ve will detect tritium (beta emitter)
+ve can use biodegradable solvents
+ve counts large batches of samples
+ve generally very accurate
+ve high efficiencies (30-100%)
Cons of liquid:
-ve bulky equipment (not portable)
-ve expensive
-ve regular calibration required
-ve no in-situ counting (only lab)
-ve quenching
How do semiconductor radiation detectors work and pros and cons?
Two main types use silicon or germanium semiconductors
When ionising radiation interacts with the semiconductor, it generates electron-hole pairs by ionising atoms in the material. These pairs move under the influence of an electric field which creates an electric signal.
PROS:
+ve high electron densities (unlike gas chamber)
+ve very sensitive (can identify isotopes)
+ve electron/hole energy low (2.9eV for Ge)
+ve pulse height proportional to energy of radiation
+ve analysis of unknown sources
CONS:
-ve very expensive (crystals and electronics)
-ve requires cooling (Ge to liquid nitrogen temps)
-ve Require long setup times
-ve generally not portable (can have some portable ones)
What are the main personal monitoring techniques?
What are the 4 main Personal monitoring techniques?
- Thermoluminescence Detectors - TLD
- Optically Stimulated Luminescence - OSL
- Film badges - original (being phased out)
- Personal radiation detectors - PRD (EPD)
How does a thermoluminescence Detector work and it’s pros and cons?
Similar mechanism to scintillation detectors
Uses LiF or CaF crystals
Incoming ionising radiation excites electrons and traps them within the crystal
When heated above 200C, the stored electrons drop back to valence band emitting a flash of light - dose received given by a the glow curve which is proportional to radiation absorbed over time period — heating also resets the device
PROS
+ve compacted device
CONS
-ve accuracy poor for low doses
-ve more expensive than other devices
Briefly describe film badges and their pros and cons
Photographic emulsion of silver halide deposited onto a plastic sheet which blackens when exposed to radiation — density of blackened area measures dose received — filters can discriminate between types of radiation
PROS
+ve cheap
+ve permanent record
+ve sensitive to low energy gammas <20KeV
+ve can discriminate between different types
CONS
-ve messy developing chemicals
-ve not as accurate as other techniques
-ve degradation of film
-ve single use
Briefly describe optically stimulated luminescence (OSL) and it’s pros and cons
Similar concept to TLDs which ‘trap’ electrons
Al2O3 scintillation crystal is heated via a laser (material and heating method main difference) | luminescence proportional to exposure | filters used for type discrimination | imaging filter used to determine static or dynamic exposure
• photon sensitivity 5KeV - 40MeV
• beta sensitivity 150KeV - 10MeV
Exposure range 10microSv - 10Sv
PROS:
+ve can be re-measured (allows archiving)
+ve more accurate than TLD
+ve large exposure range
+ve long shelf life
CONS:
-ve more expensive than TLD
-ve 2 month wearing period min
Briefly describe Electronic Personal Dosimeters (EPD) and provide pros and cons
(Most common)
Small portable devices providing an instant reading | typically use GM tube or solid state (silicon) | some display current exposure and accumulated exposure can be downloaded
PROS:
+ve real time monitoring
+ve alert if dangerous exposure reached
CONS:
-ve not very accurate
-ve specific to Cs-137
-ve can be expensive
-ve battery life can affect usability
