1. Basics:
ß- particles interact with scintillation system to produce small light flashes which are detected by photomultiplier tube and recorded as counts.
ß- particles themselves are poorly detected by photomultiplier tube and energy of ß- must be converted to light energy.
2. components:
i) radioactive material
ii) solvent
iii) 1 or more organic fluorescent materials
3. Process
i) ß- + solvent (S) to S* + ß-
(less energy)
ß- particle
excites solvent molecule to higher energy state and loses energy itself.
ii) S* + F1 to S + F1*
Excited solvent interacts with primary fluor and transmits energy to
F1.
S decays to ground state.
iii) F1* to F + hv1.
F1 decays to ground state releasing photon (hv1)
which is detected by PMT.
or
F1* + F2 to
F1 + F2*
If hv1 is poorly detected by PMT a secondary fluor may be introduced.
iv) F2 * to F2 + hv2
hv2 is detected by PMT
Photons react with PMT to produce voltage pulse which is registered as a disintegration event.
Energy of voltage pulse reflects energy of initial ß- particle released.
Counter can be set to measure voltage pulse of particular energy range.
Energy spectrum of voltage pulses
By adjusting discrimination settings it is possible to count any portion of spectrum desired.
This allows 2 isotopes to be determined in same sample.
Channel 1 – registers 3H and a portion of 14C
Channel 2 – register a portion of 14C uncontaminated by
3H.
By correcting for 14C in channel 1 it is possible to obtain value for 3H.
Counting error
- counting error reflects fact that decay is a random event.
A different number of decay events occur in any unit of time.
In general the more counts registered the more accurate is the number.
e.g. count sample for different 1 min intervals
1st count 103
2nd count 97
3rd count 98
4th count 101
etc.
The standard deviation of counts detected = the sq rt of total number of counts detected
95.5% of all counts will fall with 2 S.D.
counting error = 2 x sq rt of total number
of counts
Example of counting error
A sample with 1000 cpm is counted for 1 min
Error = 2 x sq rt 1000 = 64
% error = 64/1000 x 100 = 6.4%
i.e. count = 1000 +/- 64 cpm
A sample with 100 cpm is counted for 10 min. Total counts = 1000
Error = 2 x sq rt 1000 = 64
% error = 64/1000 x 100 = 6.4%
i.e. count = 1000 +/- 64 cpm
Counting efficiency
# of decay events registered by counter is less than total. Counting efficiency is less than 100%.
e.g. if sample undergoes 150000 dpm and instrument records 115000 cpm,
then counting efficiency
= 115000 / 150000 = 77%
why : 1) electronics
2) transfer of energy within system
3) colour quenching
- coloured material in sample absorbs photons before they react
within PMT
4) chemical quenching
- substances in mixture interfere with transfer of energy between components
of system
5) point quenching
- if sample not completely dissolved particles may be abosrbed
before they react with solvent
Correcting for chemical quenching
A. Effects of quenching
i. Total # of recorded disintegration events decreases
ii. Energy spectrum of events shifts to lower energy levels
B. Methods of correction
i. Internal standard
- add known amount of radioactivity to all samples and recount
e.g. add 100,000 dpm to sample
cpm
unquenched
quenched
sample
350
262
sample + st.
90,350
75,260
cpm standard
90,000
75,000
counting efficiency
90%
75%
ii. Channels ratio method
- based on change in energy spectrum in quenched samples
- distributed of energy spectrum depends upon degree of quenching
- the more quenching, the greater the shift to lower energies
- ratio of counts in B/A increases as quenching increases
- set up a series of vials with constant dpm and add increasing amt of a quenching agent – e.g. chloroform
- determine cpm, counting efficiency and channel ratio (B/A) for each vial
- plot counting efficiency vs channel ratio
e.g. (question #4 p49a in lab manual)
11000 dpm 14C added to a series of tubes
chloroform (ml)
cpm A
cpm B efficiency
B/A
1 0
10,000
2,500
91%
0.25
2 0.5
8,500
2,600
77%
0.30
3 1
6,400
2,900
58%
0.45
4 1.5
5,100
2,800
46%
0.54
5 2
3,300
2,600
30%
0.79
unknown
1,250
500
~ 52% 0.40
Internal vs external standard
Internal standard - determines channel ratio using radioactivity of sample
Accuracy decreases if sample contains low counts
External standard
- determines channel ratio using source of radioactivity outside of
sample
- emitter, highly radioactive e.g. 137Cs
- accurate, easy, method most commonly used