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

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

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

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

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

Electron Capture Proton + Electron : Neutron + X-Ray Z’ =Z-1 ,A= constant 125 53I: 125 52 Te+ X- Ray Pair production Photon anhilated: Electron + Positron

Alpha Emission Z’=Z-1, A’=A – 4 22688Ra : 22286 Rn + 42He-------20656Pb Toxic Z<60 less likely to produce He Health risk: highly ionizing Lose energy by collision Low penetration

Emission of gamma rays Nuclear transformation Z’=Z ,A= Constant 131 53I = 131 54Xe + Beta- + gamma Source : nuclear (X- rays are extranuclear) Characteristic of each nuclues Highly penetrating

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

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

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

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

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

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

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)

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

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

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

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

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

Cell harvestor

Beta counter

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

Gamma counter