1. RADIATION COUNTER Introduction
Radioisotopes: spontaneous disintegration with emission of EM radiation (fission)
Radiochemical methods:
Activation analysis: Sample made radioactive by irradiation
Isotopic dilution: Radioactive subs mixed with known amount of sample  measurement

2. Radiochemical methods (cont…)
Radiometric analysis: Radioactive reagent used for separation of component of interest activity measured
Tracer Analysis: Radioactivity induced by adding radioactive species in sample
Tagging : Weighed amount of radioactivity tagged analyte added to measured amount of sample

3. Isotopes of biological importance
3H, 14C, 35S, 32P, 125I, 131I, 51Cr
Toxicity and energy increases
3H, 14C : Organic comp. Tagging
35S : amino acids
125 I & 131I : Clinical studies

5. Types of decay Negatron Emission
Neutron: Proton + Negatron (nuclear origin)
Z’= Z +1
14 6C:14 7N+Beta-
Negatron+Electron=2Gamma rays
Not effective in producing ion pair
Greater penetration

6. POSITRON EMMISION Proton : Neutron + positron Z’=Z-1, A constant
22 11Na:2210Ne+beta+
Positron emission tomography (PET): Brain scanning technique to identify active and inactive portions

7. Electron Capture Proton + Electron : Neutron + X-Ray
Z’ =Z-1 ,A= constant
I: Te+ X- Ray
Pair production
Photon anhilated: Electron + Positron

8. Alpha Emission Z’=Z-1, A’=A – 4
22688Ra : Rn + 42He Pb
Toxic
Z<60 less likely to produce He
Health risk: highly ionizing
Lose energy by collision
Low penetration

9. Emission of gamma rays Nuclear transformation Z’=Z ,A= Constant
131 53I = Xe + Beta- + gamma
Source : nuclear (X- rays are extranuclear)
Characteristic of each nuclues
Highly penetrating

10. Radioactive decay rates
-dN/dt=λN N=No. of radioactive nuclei at time t
λ= Decay constant
N=Noe- λt
T1/2=ln2/ λ =o.693/ λ
Activity A=-dN/dt

11. Units of radioactivity
SI Unit:Bequerel(disintegration/sec)
1 curie: The quantity of radioactive material in which disintegration same as 1g radium
1 Cu=3.7X107Bq
Specific activity:Amount of radioisotope (dps/dpm or cpm/cps)) per unit mass of mixture expressed in moles or gram

12. Advantage Good Accuracy Widespread availability
Chemical separation avoided
Counting of pulse of electricity produced when particles strike detectors

13. Detectors for measurement of Radiation
3 detection methods
(i) Ionization of gas eg. Ionization chamber,
Proportional counters & Gieger muller counter
(ii) Excitation of solid or solution eg Scintillation counter
(iii) Ability of radioactivity to expose photographic emulsion eg Autoradiography

14. Methods based on gas ionization
High energy particles Gas Ion electron
Particles ability charge & mass
α > β > γ (10000:100:1)
Current depends on: Voltage & no. of radiation particles

15. Methods based on gas ionization
High energy particles Gas Ion electron
Particles ability charge & mass
α > β > γ (10000:100:1)
Current depends on: Voltage & no. of radiation particles

17. Regions of gas ionization chamber
(i) Ohm’s law followed (V=IR)
(ii) Ionization chamber region (1 ion pair per collision)
(iii)Proportional counter region (secondary & tertiary ionization)
(iv) Limited proportional region (not used)
Geiger muller (GM) region: complete ionization of all gas particles
(vi) Continuous discharge:Ion induced avalanche Quenching agent (Ethanol & halogens, etc)

18. Ionization chamber Low current Need sensitive device for detection
Used to study property of radiation by electroscope
Electric field applied between 2 electrodes across
Potential adjusted to minimum recombination of ion pairs without causing recombination
Quick acting detectors
Pulse proportional no. of electrons releases by ionization radiation
Air used for alpha particles
Xenon/krypton used for X/gamma rays

19. Proportional counters
Ionized electron move towards electrode resulting in sec. & tertiary ionization
Pulse output increased but prop. to initial ionization
No. of sec. Electrons prop. to no. of pri. Elec.
Townsend avalanche effect (Multiplication factor:10-105)
Depends on voltage, gas pressure and counter dimension
Discharge depends on immediate environment of ionizing particle, path of ion pair, sec. electrons and positive ions

20. Gieger muller counters
Complete ionization induced
Current size independent of primary ions produced
Not possible to differentiate different isotopes
Not for quantitative estimation
Radiation monitoring
Contamination monitoring: presence or absence of activity

21. Photographic emulsion
Distribution of radioactive material in thin section.
Autoradiography (PAGE)
Film badges: Radiation exposure monitoring

22. Example in research
Beta counter : To study lymphocyte cell proliferation by tritiated thymidine uptake method
Steps
Cell separation
Stimulation by PHA
Labelling by tritiated thymidine
Cell harvesting
Counting by beta counter

23. Cell harvestor

24. Beta counter

25. Example in research (cont…)
gamma counter: Estimation Natural killer cell mediated cytotoxicity by chromium release assay
Preparation of effector cells
Preparation of target cells- Cr-51 labelling
Cytotoxicity assay
counting

26. Gamma counter