Half-life

• It's impossible to predict when a specific atom is going to decay, but you can predict the number of atoms that will decay in a certain time period.

• The halflife is the amount of time it takes for half of the atoms in a sample to decay. The halflife for a given isotope is always the same ; it doesn't depend on how many atoms you have or on how long they've been sitting around.

Carbon-dating• Carbon has 3 isotopes, 2 of which are stable

(carbon-12 and carbon-13) and one which is radioactive (carbon-14). Of these isotopes, the most common in nature is carbon-12.

• Production of Carbon-14• Carbon-14 is produced in the atmosphere by the

interaction of neutrons produced by cosmic rays with the stable isotope of nitrogen, nitrogen-14:

HCNn 11

146

147

10

• The carbon-14 atoms produced are then incorporated into carbon dioxide molecules to produce 14CO2 molecules which mix with the most common 12CO2 molecules in the atmosphere.

• The 14CO2 enters plant tissue as a result of photosynthesis or absorption through the roots.

• 14C enters animal tissue when animals eat plants containing 14C.

• The amount of 14C produced in the atmosphere is balanced by the continual decay of 14C to produce 14N and a beta-particle:

eNC 01

147

146

• When a plant or animal dies it stops taking in carbon-14 and radioactive decay begins to decrease the amount of carbon-14 in the tissues.

• The age of the plant or animal specimen containing carbon, such as wood, bones, plant remains, is determined by measuring the ratio of carbon-12 to carbon-14.

• The half-life of carbon-14 is 5730 years, because of its relatively short half-life, carbon-14 can only be used to date specimens up to about 45,000 years old. After this the amount of carbon-14 present in the sample is too small to be measured precisely.

• Carbon-14 can not be used to measure the age of very young specimens as the change in the ratio between the amount of carbon-12 and carbon-14 will not be sufficient to be detected.

• The tube is filled with Argon gas, and a voltage is applied between the wire and the case.

• When a particle enters the tube, it ionizes an Argon atom. The electron is attracted to the central wire, and as it rushes towards the wire, the electron will knock other electrons from Argon atoms, causing an “avalanche” (gas amplification)

• •Thus one single incoming particle will• cause many electrons to arrive at the wire, •creating a pulse which can be amplified and• counted. This gives us a very sensitive detector.

Geiger Counter

Proportional Counter

• Detector and amplifier (gas amplification)

• Radiation causes primary then secondary ionization - but just in small region of tube.

• For G-M -ionization of whole tube

• The smaller amount of ionization in the proportional counter allows for smaller dead-times and for us to have proportional counting. But higher LOD than G-M

Proportional Counting

• Pulses are fed into a counter (pulse height discriminator) which requires that the pulses be of a certain height before it will count them.

• The height of the pulse depends on the energy lost by the particle.

• Hence can distinguish β’s from different sources

Semiconductor detector

• Based on ionization of semiconductor causes formation of e--hole pairs in

depletion zone of reversed-biased p-n junction

• Pulse size (current produced) is proportional to energy lost by (~3.6 eV/ion pr in Si)

• Good energy resolution

• Can only be made fairly small

NaI Scintillation Counter

• Single NaI crystal is grown from molten salt.

• Can be up to a 25 cm NaI crystal (big = $$$)

• O.5 mole % TlI is added as activator

• Ionizing radiation produces a flash of light 330-500 nm

• Detect with PMT

• Size of pulse is proportional to energy lost

NaI Scintillation Counter

• Good for stopping high-energy rays.

• I has larger Z than Ge - more efficient at stopping rays.

• Energy resolution is not as good as with a semiconductor detector but the counting efficiency is higher

Liquid Scintillation counting• Good for counting low energy ’s (tritium

and C-14) and ’s.

• Scintillation cocktail - eg toluene and p-terphenyl and 2,5-diphenyl oxazole

• Energy from excites solvent. Energy is then transferred to primary fluor then secondary fluor then get flash of light

• Coincidence counting with two PMT’s to overcome interference by dark current

LiquidScintillationCounting

Liquid Scintillation counting

• Pulse height is proportional to the energy adsorbed from the radiation

• Can set the pulse height discrimination in several channels so you can count several ’s at once

• eg 14-C and 3-H

Cerenkov counting

• Bluish white light 300 - 700 nm

• It is emitted when an e- passes through the medium at a speed greater than the speed of light in that medium

• In water,the lowest energy particles that will cause this are 0.26 MeV

• ie need a hard beta

• P-32, K-42, Sr-90

Cerenkov counting

• Use polyethylene vials because glass would absorb

• Advantage over scintillation counting is it can be done in water and is cheaper and less complicated

• Disadvantage - no good for soft betas from C-14 and tritium.