1. Measurement and Detection of Ionizing Radiation
Methods of detection
Electronical instruments

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. The key components of an ionization chamber are shown here
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.
Ionization Chamber

7. Radiation ionizes the gas
Radiation ionizes the gas. Ions move toward electrodes, creating current.

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

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

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”
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

17. Liquid Scintillation counters
Workhorse in biology labs for many years
Very useful for beta emitters, some alpha
Modern equipment:
Computer driven

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.