1. Detecting and Measuring Ionizing Radiation -2
Day 3 – Lecture 4

2. Objective
To give an understanding of the Monitoring instruments and survey meters for measuring ionization for contamination detection purposes and measuring the dose rates.

3. Contents Monitoring instruments; Types of dosimeters;
Survey instruments;
Correct use of instruments;
Spectroscopic analysis.

4. Choice of monitoring instrument…
… depends on...
High or low levels?
Particles or photons?
Energy of photons?
Required accuracy?

5. Choice of detector type…
… may be important for ...
Required sensitivity
… otherwise NOT so important !
The design of the instrument to fulfill the requirements more important!

6. Contamination detectors
β - emitters GM-tube
Proportional counter
γ - emitter (< 50 keV) GM-tube
NaI(Tl) scintillation detector
γ - emitter (high energy) Proportional counter
All with an appropriate design!

7. Dose-rate meters γ - emitter GM-tube Ionisation chamber
Scintillation detector
Often designed to meet the requirements to measure one of the operational dose quantities defined in ICRU 47

8. Change scale if necessary
Making Measurements
Switch on before entering
Check batteries
radiation area
Move monitor slowly
Change scale if necessary

9. WIPE TEST Wipe a known surface area with an absorbent
material moistened with water or alcohol. Put
the sample in a tube and measure the activity
in a well counter or a liquid scintillation counter.
(cps-BG)/(Ec*Ew*A) = contamination (Bq/cm2)
cps: counts per second for sample
BG: instrument background
Ec: counter efficiency (cps/Bq)
Ew: swipe efficiency (assumed to be 0.1)
A: area swiped (cm2)

10. Dose Rate Measurement
Reliable measurements are necessary in order to control radiation exposure.
This is achieved through the use of a range of radiation instrumentation which measure accumulated radiation doses and/or dose rates.

11. Film Badges
Film badges are passive devices that use special photographic film to record any accumulated radiation exposure received over a period of time. Provided the appropriate holder is used, they can measure doses from beta, x, gamma and neutron radiation exposure.
Advantages: cheap, provide a permanent record.
Disadvantages: not robust (can be affected by water, humidity).

12. Thermoluminescent Dosimeters (TLD)
TLDs typically are based on the use of lithium fluoride which traps the energy received from ionizing radiation. When heated during the assessment process, the trapped energy is released as light. The amount of light released is proportional to the radiation dose.
Depending on the intended use, the lithium fluoride may be in the form of small crystal chips, rods or discs. e.g. finger dosimeters.

13. Optically Stimulated Luminescence Dosimeters (OSL)
OSLs are based on the use of aluminium oxide and, like TLDs, trap the energy received from ionising radiation. They are assessed by exposing the OSL to laser light.

14. Direct Reading (‘pocket’) Dosimeters
A ‘pocket’ dosimeter is a small pencil-like electroscope which is charged (set to zero) in an external device. An internal scale showing the radiation dose received can be viewed though an eyepiece.
Electroscope dosimeters can be subject to spurious readings and do not normally tolerate physical shock well.

15. Electronic Personal Dosimeters (EPDs)
Electronic Personal Dosimeters (EPDs) are small electronic dosimeters that use a battery to power a detector to measure the accumulated dose. These devices are widely used in industrial and medical applications. They can include a dose rate function and an alarm for pre-determined radiation dose rates.

16. Survey Meters
A survey meter is used to measure radiation dose rates (sometimes only as an indicative count rate).

17. Survey Meters
There is a wide range of survey meters available, from simple geiger detectors through ionization survey meters to neutron detectors.
To accurately measure radiation dose rates, survey meters must both respond to the type of radiation under investigation and be calibrated for the radiation energy (eV).
It is therefore important that the correct monitoring equipment for any given type of radiation is used. Not doing so may result in failure to detect, or erroneous readings.

18. Survey Meters
The simplest meter, the geiger, is very sensitive to even low radiation dose rates but unless suitably compensated, can give very misleading readings at lower radiation energies e.g. x-rays.
In very high dose rates, survey meters must continue to respond. Some survey meters may “fold back” and read zero in very high level radiation fields.
Equipment suitable for customs purposes will be discussed later.

19. Radiation Detection and Dose Rates
Equipment that electrically generates ionizing radiation (e.g. x-ray apparatus, linear accelerators etc.) do not emit radiation unless assembled and energized.
i.e. the cross-border movement of such apparatus (say from supplier to end-user) does not pose a radiation hazard to Customs officers.

20. Radiation Detection and Dose Rates
For properly packaged radioactive materials, the international transport regulations prescribe different radiation dose rates for different types of transport packaging.
Permitted dose rates generally range from 0 to 2,000 µSv h-1 at the surface of the package and up to 100 µSv h-1 at 1 metre from the surface of the package.
Higher dose rates are permitted in special circumstances which will be dealt with later.

21. Spectroscopic Analysis
During the radioactive decay process every radioisotope emits radiation with distinct energies. The spectrum of energies can be analysed to determine the identity of the radioisotope (or a mixture of radioisotopes) that may be present. Portable instruments are available for this type of analysis.
Ortec
Canberra

22. Where to Get More Information
International Atomic Energy Agency, Postgraduate Educational Course in Radiation Protection and the Safety of Radiation Sources(PGEC), Training Course Series 18 (2002), IAEA, Vienna (2003)
International Atomic Energy, Training on Radiation Protection in Nuclear Medicine, Module 02 on Radiation Physics, IAEA, Vienna (