Radiation Protection Instrumentation Flashcards
What is the difference between the quantities; air kerma, H*(10), Hp(10), Hp(3) and Hp(0.07)? Give examples of when these quantities might be measured and possible units.
- Air kerma is the kinetic energy released per unit mass of air due to ionisation. It is a physical quantity and is typically measured with an ionisation chamber in units of J/Kg or Gy.
- H*(10) is ambient dose equivalent - the operational dose quantity for area monitoring of strongly penetrating radiation. This could be measured with a survey meter in microSv/hr, for example.
- Hp(10), Hp(3) and Hp(0.07) are the personal dose equivalent operational quantities for whole body dose, dose to the lens of the eye and dose to the skin or extremities, respectively. The numbers represent the appropriate depth within the body. These quantities would be measured with personal dosimetry in mSv.
What are the three types of detectors typically used in radiation protection?
- Gas-filled detectors.
- Scintillation detectors.
- Semiconductor (solid state) detectors.
How is detection efficiency different for alpha/beta detectors and gamma/x-ray/neutron detectors?
- Alphas and betas only need to travel a short distance before interaction and, therefore, detection. This means it is likely the detector detects every charged particle entering the active volume and a counting efficiency of 100% is possible.
- For gamma/x-ray/neutrons, large distances may be travelled before any interaction. This means the efficiency is often less than 100%.
What is the absolute efficiency and intrinsic efficiency of a detector?
- Absolute efficiency = No. of pulses recorded/No. of radiation quanta emitted by source. This, therefore, depends heavily on the setup geometry (e.g. source-to-detector distance) alongside detector and radiation properties.
- Intrinsic efficiency = No. of pulses recorded/No. of radiation quanta incident on detector. This, therefore, depends on on detector properties (e.g. thickness of detector in direction of incident radiation) more, as well as setup geometry and radiation properties.
What is the difference between total efficiency and peak efficiency?
- Total efficiency: All detected interactions are counted towards the efficiency calculation.
- Peak efficiency: Only interactions depositing the full energy of the incident radiation are counted towards the efficiency calculation.
What is detector dead time? What is the effect of this?
- The minimum amount of time between two detection events such that they are recorded as two separate pulses. This could be due to the detector or electronics.
- Due to the random nature of radioactive decay, it is possible that an event may be lost due to this dead time. Corrections, therefore, must be applied for this.
What are paralysable and non-paralysable systems?
- Paralysable: Each detection event occurring during the dead time of the detector will not be recorded as a count but will restart the dead time. Eventually, no events would be recorded at all for high count rates in this type of system.
- Non-paralysable: Events occurring during the dead time of the detector are lost and have no affect on the behaviour of the detector.
What is the energy resolution of a detector?
- This is the ability of the detector to distinguish between two incident radiations of different energy. This can be determined from a plot of the number of pulses per pulse height vs pulse height using the following equation:
R = FWHM/H0 where FWHM is the full width half maximum of the resultant peak (equal to 2.35sigma) and H0 is the average pulse height. - Therefore, the lower the value of R, the better the energy resolution of the system. More fluctuations from pulse to pulse will correspond to a poorer energy resolution.
What is the typical averaging period for most handheld count-based instruments to ensure uncertainty in the estimated count rate is acceptable?
Typically 5 s for lower count rates and 0.5 s for higher count rates. This takes into account the Poisson statistics associated with radiation (i.e. sigma = sqrt(N)).
Explain the regions of operation for gas-filled detectors referencing a pulse height vs bias voltage plot.
- Recombination region: At low bias voltages, some ions created by the incident radiation will recombine before reaching the electrodes and so will not be detected. This effect will become less apparent as bias voltage is increased and pulse height will increase linearly.
- ion chamber region (after bias voltage is further increased): Recombination will cease and all ions will be collected. This will result in a flat response in which the resultant pulse height is independent of small variations in bias voltage but dependent on the energy of incident radiation.
- Proportional region: Further increases in bias voltage result in avalanche multiplication with pulse height becoming linearly dependent on bias voltage.
- Limited proportionality region: This linear proportionality will than begin to change as cascading ionisations begin.
- Geiger-Muller region: In this region, cascading ionisations become the major effect. The large signals apparent can be useful for radiation counters. Signal is not proportional to the energy of incident radiation.
- Eventually, continuous discharge will become apparent whereby a large number of cascade avalanches occur in a short period of time. This results in a build up of positive charge near the anode, reducing the electric field and terminating the Geiger discharge.
Briefly describe how an ionisation chamber works. What does an ionisation chamber measure and how is this converted to a dose value?
- Chamber containing air at normal atmospheric pressure.
- Outer (chamber) and central electrode held a few 100 V potential difference.
- Incident radiation ionises gas.
- Resultant ion pairs are collected at electrodes.
- Resultant current proportional to radiation dose. It also depends on the mass of air irradiated.
What does an ionisation chamber measure and how is this converted to a dose value? How is this conversion applied in practice? What corrections are required for calibrated field instrument measurements?
- Specific charge (C/Kg) is measured. This can be converted to air kerma (J/Kg) by multiplying by the average energy required to create an ion pair (J/C).
- In practice, air kerma conversions are established in primary standards laboratories using a free air ionisation chamber for which the mass of air is precisely known.
- Further corrections are applied for beam quality, fraction of the beam lost to Bremsstrahlung, field distortion, air attenuation, recombination, scattered photons etc.
- This calibration can be applied to a secondary standard which can then be used to calibrate field instruments.
- Field instruments still have to be corrected for temperature and pressure where absorption efficiency changes depending on these parameters due to the changes in volume of air. The ideal gas laws (P1V1/T1 = P2V2/T2) can be used to determine the correction where it is assumed gas volume is proportional to the current produced.
List some pros and cons of ionisation chambers.
Pros:
- Linear response across large dynamic range.
- Variable chamber size for variable sensitivity/spatial resolution.
- Good for standards cross-calibrations.
- Can measure dose rate or accumulated dose.
Cons:
- Electrometers required for readout are limited by leakage current.
- Temperature/pressure corrections required.
Give some examples of uses of ionisation chambers.
- Survey meters.
- AECs.
- Beam outputs (e.g. Farmer chamber for RT, Radcal PDC for DR etc.).
- Dose calibrators.
How does proportional counter work?
- Proportional counters operate in the proportional region, using increased bias voltage when compared to ionisation chambers.
- The increased bias voltage means the ion pairs created from incident radiation have enough energy to create an avalanche of further ionisations.
- Each avalanche created is independent of those created from other initial events.
- They can, therefore, allow for energy discrimination.
