Measurement and Detection of Ionizing Radiation

1. Measurement and Detection of Ionizing Radiation First lecture

2. Ionizing radiation is invisible • Many methods are available for detection and measurement, including • Ionization detectors • Scintillation detectors • Biological methods • Thermo luminescence • Chemical methods – free radicals produced • Measurement of heat- energy dissipated

3. Ionization • All detecting methods are based on the interaction of the radiation with matter. If a radiation does not interact with any matter, we have no method of the detecting it. Since ionization is an important process for radioactivity, most detectors exploit the signals generated due to ions and electrons . • on pairs in a gas produced by ionizing radiation do not recombine until the energies of electrons have dissipated. In a gas, ions and electrons move freely

4. Ionization • Devices contain a gas that can be ionized • A voltage is applied to the gas • Specific instrumentation and types of measurement depend on amount of voltage applied to the gas. • Three types of instruments: • Ion chambers • Proportional counters • Geiger-Mueller counters

5. Log of electrical signal vs. voltage

6. Ionization Chamber • The key components of an ionization chamber are shown here. It consists of a detector chamber, a voltage supplier (battery), an ampere meter, and a load resister . Ionizing radiation enters the detector chamber and ionizes the mixture of gas in it. The electrons drift towards the positive electrode and ions move towards the negative electrode. Thus, ampere meter detects a current. • The number of ion pairs is proportional to the number of ionizing particles entering the detector chamber. Thus, the current is proportional to the intensity of ionizing radiation. • The light electrons drift 100,000 times faster than the heavy ions. The motion of electrons is mostly responsible for the current.

7. Radiation ionizes the gas. Ions move toward electrodes, creating current. http://www.science.uwaterloo.ca/~cchieh/cact/nuctek/interactdetector.html

8. Ion chamber continued • Voltage is high enough that ions reach the electrodes, produce current. • Proportional to energy: the more energy, the more current. • Generally requires some amplification of the signal. • Example of use: pocket dosimeters http://www.ludlums.com/images/dosimeter.jpg

9. Proportional Counters • At some hundreds volts, the improvement in sensitivity is more than collecting all the When voltages applied to electrodes of ionization chambers increase, the sensitivities increase. Electrons and ions on the electrode. The currents corresponding to multiples of ions and electrons produced by radioactivity. To distinct them from simple ionization chambers, these detectors are called proportional counters. • In proportional counters, the high voltage applied to the electrodes created a strong electric field, which accelerate electrons. The electrons, after having acquired the energy, ionize other molecules. Production of secondary ion pairs initiates an avalanche of ionization by every primary electron generated by radiation. Such a process is called gas multiplication. • The gas multiplication makes the detection much more sensitive. Yet, the current is still proportional to the number of primary ion pairs. • When voltages applied to proportional counters get still higher, sparks jump (arcs) between the two electrodes along the tracks of ionizing particles. These detectors are called spark chambers, which give internal amplification factors up to 1,000,000 times while still giving an initial signal proportional to the number of primary ion pairs.

10. Proportional counters Each ionization electron is accelerated by the voltage so that it ionizes more of the gas. The higher the energy of the radiation event, the greater the avalanche, the higher the current Each ionization event detected separately. -+-+-+-+-+-+-+ -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .

11. Geiger Mueller counters http://www.pchemlabs.com/images/eberline-rm20-geiger-counter-a.JPG

12. How Geiger counters work • Voltage is high enough that every radiation event triggers a complete avalanche of ionized gas • Does not discriminate among different energy levels • Each event is registered • A quenching agent stops the reaction, resets gas for next event • Slow response time (comparatively) but simpler circuitry. • Good for simple, sturdy, instruments • Best for gamma; low efficiency for alpha, beta.

13. More Geiger details Higher voltage leads to constant avalanches; instrument “pegs”. Improved efficiency with pancake probe: collects more radiation due to geometry.

14. Proper use of Geiger counters as “survey meters” • http://www.orau.gov/reacts/measure.htm • Units of radioactivity and radiation • Radiation detection instruments and methods • First check battery and check source • Enclosed radioactive material of known amount • Check level of background radiation • Survey area in question • Move survey instrument slowly • Keep constant distance from object being surveyed; do not make contact.

15. Solid scintillation counters • Crystal-based • Radiation hits crystal which releases visible photons • Photons amplified by photomultiplier tube, converts to electrical signal • Zinc sulfide • Good detection of alpha particles, rapid response time • Sodium iodide w/ thallium • Good for detection of gamma

16. http://www.fnrf.science.cmu.ac.th/theory/radiation/Radiation%20and%20Radioactivity_files/image018.gifhttp://www.fnrf.science.cmu.ac.th/theory/radiation/Radiation%20and%20Radioactivity_files/image018.gif

17. Liquid Scintillation counters • Workhorse in biology labs for many years • Very useful for beta emitters, some alpha • Modern equipment: • Computer driven http://www.gmi-inc.com/Genlab/Wallac%201414%20LS.jpg

18. Basic principles • Radioactive sample is mixed with organic solvents (cocktail) • Toluene replaced with biodegradable solvents • Detergents allow up to 5% aqueous samples • Radiation hits solvent, energy is absorbed by solvent; Energy passed to one or more fluors • Fluor emits visible light which is detected • By fluorescence • Amplified by photomultiplier, converted to electrical signal.

20. Coincidence circuitry • Photomultipliers very sensitive • Inside of instrument completely dark • Tubes give off “thermal electrons” • Result would be very high background counts • Coincidence circuitry compares results from 2 photomultipliers • Event not detected by both: thermal electron • Ignored • Event detected by both is affect of beta particle • Counted.

21. Counts and energy discrimination • As radiation travels through solvent, it gives up energy • The more energy it has, the more fluor molecules get excited and release photons • Thus, the higher the energy, the brighter the flash • The higher the electrical pulse sent from the PMs • Instruments can be electronically adjusted • Discriminators set for different “pulse height” • Able to count betas from H-3 vs. C-14 vs. P-32

22. Beta energy spectra cpm Pulse height

23. Summary of capabilities • Pulse height • From brightness of flash; the more energetic the radiation, the brighter the flash. • Discriminators (“gain”) in the instrument can be set so you determine what energy you want counted. • Number of pulses • Corresponds to how many flashes, that is how many radiation events (decays): the amount of radioactivity.

24. Difficulties with LSC • Static electricity: causes spurious high counts, esp. when humidity is low; • don’t wipe outside of vials! • Chemiluminescence: chemical reactions in sample, from overhead lights, glass. • Suspiciously high counts can be redone; chemi-induced high counts subside over time. • Quench • Anything that interferes with counting efficiency. • Measured: counts per minute (cpm) • Desired: decompositions per minute (dpm)

25. Counting efficiency • Because samples are usually dispersed in clear containers, geometry is favorable for energy transfer in all directions and good light emission • Not all decay events will get registered, however, because no system is 100% efficient • We seek to know the # of decompositions per minute (dpm) but measure the counts per minute (cpm). • Using standards helps determine efficiency.

26. Effect of Quench

27. All about quench • Chemical quench • Acids, bases, high salt, any chemical that interferes with transfer of energy from the solvent to the fluor. • Result: fewer activated fluor molecules, less intense flash, interpreted as a lower energy event. • Color quench • Colored material absorbs visible light from fluor • Less intense flash, appears as lower energy event

28. About quench -2 • Self absorption • If particulate matter not well suspended, energy not absorbed by fluor, not detected as well. Both lowering of cpm and forcing into lower energy range.

29. Counting statistics • Radioactive decay is a random event • To be sure results are reliable, a minimum number of decay events must be recorded. • Reliability depends on total number of counts! • Example • Statistical significance is the same in these two cases; • 10 minute count yielding 500 cpm • 1 minute count yielding 5000 cpm. • Both have total of 5000 counts • Instruments have settings for stopping count when a certain statistical threshold is reached.