Measurement and Detection of Ionizing Radiation

1Measurement and Detection of Ionizing Radiation

• Ionizing radiation is invisible• Many methods are available for detection and

measurement, including– Ionization detectors– Scintillation detectors– Biological methods– Thermoluminescence– Chemical methods – free radicals produced– Measurement of heat- energy dissipated

2Ionization

• 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

3Log of electrical signal vs. voltage

4

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

http://www.science.uwaterloo.ca/~cchieh/cact/nuctek/interactdetector.html

5Ion 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

6Proportional 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.

• Used for neutrons

-+

-+-+

-+-+-+-+

-+-+-+-+-+-+-+-+

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .

7Geiger Mueller counters

http://www.pchemlabs.com/images/eberline-rm20-geiger-counter-a.JPG

8How 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.

9More Geiger details

Higher voltage leads to constant avalanches; instrument “pegs”.

Improved efficiency with pancake probe: collects more radiation due to geometry.

10Proper use of Geiger counters as “survey meters”

• http://orise.orau.gov/reacts/guide/index.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.

11Solid 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

• New ones showing up

12

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

13Liquid 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

14Basic 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.

15

16Coincidence 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.

17Counts 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

18

Beta energy spectracpm

Pulse height

19Summary 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.

20Difficulties with LSC1. Static electricity: causes spurious high counts,

esp. when humidity is low; 1. don’t wipe outside of vials!

2. Chemiluminescence: chemical reactions in sample, from overhead lights, glass.

1. Suspiciously high counts can be redone; chemi-induced high counts subside over time.

3. Quench1. Anything that interferes with counting efficiency.

1. Measured: counts per minute (cpm)2. Desired: decompositions per minute (dpm)

21Counting 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.

22Effect of Quench

23All 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

24About 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.

25Counting 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.