Chris - Radiation Detectors Flashcards

1
Q

What are the three ways EM radiation produces ion pairs in matter? How do they differ?

A
  1. 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

  1. 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

  1. Pair Production
    “ if gamma ray energy is&raquo_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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Biggest difference between X-ray and Gamma ray photons?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What are the most important measurements to make for the nuclear industry?

A

The energy (principle via calorimetry) and number of events

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is an ion chamber and list it’s pros and cons

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is a Geiger Muller Tube and state its pros and cons

A

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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is a smear counter and what’s it used for?

A

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 well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

How are neutrons detected?

A
  1. Cause fission reactions to occur on very small samples of uranium
  2. Diamond radiation detectors, absorbs neutron and ejects alpha particle which is detectable

**hard to measure directly but can be measured indirectly

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What are scintillation detectors and their liquid versions and what is the most common material of choice used?

A

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 well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

How do semiconductor radiation detectors work and pros and cons?

A

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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What are the main personal monitoring techniques?

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What are the 4 main Personal monitoring techniques?

A
  1. Thermoluminescence Detectors - TLD
  2. Optically Stimulated Luminescence - OSL
  3. Film badges - original (being phased out)
  4. Personal radiation detectors - PRD (EPD)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

How does a thermoluminescence Detector work and it’s pros and cons?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Briefly describe film badges and their pros and cons

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Briefly describe optically stimulated luminescence (OSL) and it’s pros and cons

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Briefly describe Electronic Personal Dosimeters (EPD) and provide pros and cons

A

(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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Most useful detector for tritium?

A

Low energy beta emitter
Only practical means is a wipe test
• use swab which is soluble in scintillant
• assume 10% pickup
• swab area of 100cm2
•measure in a LSC ~ 30-60% efficient (LSC must be calibrated)

GM tube could be useful for area contamination

Ion chamber more useful in a highly radioactive area (GM tube would become saturated quickly)

17
Q

What radioactive isotopes are common beta emitters and what’s the best way to detect them?

A

C-14, S-35, P-32, P-33

Portable GM mini monitor

Use shielding to determine relative energy

Also LSC for accurate results

18
Q

What factors must be considered when choosing the appropriate data acquisition system? Why is something such as gain important?

A
  1. Input signal
  2. Sampling rate
  3. Throughput
  4. Resolution
  5. Range
  6. Gain

Increase Amplitude—> increase detectability (gain increased)

Gain settings determine how much the signal is amplified before being processed by the system — weak signals can be enhanced making them more distinguishable from background noise.

19
Q

What are the main ways radiation (alpha, beta, gamma and X-rays) interact with matter? And how can detectors utilise these processes?

A
  1. IONISATION — removal of an electron from an atom forming an ion
  2. EXCITATION — addition of energy to the atom, giving an excited state

These can be exploited via:
• Ionisation in gases
• Ionisation in solids/liquids (silicon)
• Changes in chemical systems

20
Q

What are the different types of materials available in a semiconductor detector? Features of each?

A
  1. Germanium
    • requires cooling due to small band gap of 0.66eV (acts as an insulator until photons from radiation hits it)
  2. Silicon
    • Can be operated at room temp using pn junction (can manipulate band gaps)
  3. Diamond (CVD or single crystal)
    • allotrope of carbon
    • large band gap (requires no depletion zone | 5.5eV)
    • very radiation hard
    • CON - low signal and high cost

**Band Gap - lower the band gap the easier to promote electron to generate a current

21
Q

General pros and cons of semiconductor detectors

A

PROS:
1. have a High Density (compared to gas)
• large energy loss in a short distance
• can achieve positron resolution of less than 10microm (better than gas) due to smaller diffusion effects

  1. Low ionisation energy (0.66-5.5eV)
    • gas 20-40eV
    • scintillators 400-1000eV

CONS:
1. No internal amplification (small signal)

  1. High cost per surface unit
    • large power consumption- cooling sometimes required
22
Q

One of the most important parameters of a detector is the signal-to-noise ratio (SNR). What is this? What are the requirements? What is most optimal material?

A

A good detector needs an large SNR

This requires:
1. Large signal - low ionisation energy —> small band gap
2. Low noise - very few intrinsic charge carriers —> large band gap

Diamond is an excellent material, however even artificial diamond (CVD diamond) is too expensive for large area detectors

23
Q

How do energy bands compare across isolaters, semiconductors and metals?

A

An isolated atoms electrons have only discrete energy levels — in solid materials, the atomic levels merge to form energy bands.

Metals - conduction and valence band overlap

Isolaters and semiconductors - these are separated by an energy gap —> BAND GAP

(This gap is large for isolaters)

24
Q

What is the band gap in semiconductor detectors and why is it significant?

A

Band gap is the energy difference between the valence band (filled with electrons) and the conduction band (where electrons can move freely) in semiconductor material. When radiation interacts with the detector, electron-hole pairs are created.

The band gap is the energy required to create these electron-hole pairs — radiation liberates electrons from the valence band into the conduction band, leaving behind +ve charged holes.

Lower the band - lower energy radiation detected.