BGYC23H3
DETECTION OF RADIOACTIVITY
LIQUID SCINTILLATION COUNTING
 

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