Gamma Spectroscopy Software Solutions - ARPA Lombardia

44 Pages·2010·1 MB·English

Gamma Spectroscopy
Software Solutions
Version 3.2
ISOCS - LabSOCS
™ ™
Version 4.2
Cascade
Summing
Correction
Monica Gattinoni
Summing Effects Definition
Random Summing
(cid:1)
loss of observable peak area as a function increasing count rate
–
independent of energy, sample-to-detector distance, number of
–
nuclides in the sample
correctable with the use of a pulser or a stationary reference
–
source
Cascade Summing (True Coincidence Summing)
(cid:1)
loss or gain of observable peak area as a function of nuclide
–
decay scheme and geometry
independent of count rate
–
different effect for different gamma energies of even the same
–
nuclide
Distance Result, 22% Detector
1.5
a 244 keV
e
r
• Example of coincidence A

k
summing effect for some a
e 1085 keV
P
of the Eu-152 gamma
t
c
e
lines as a function of r
r
o 1112 keV
C
sample - detector distance

s. 1
v
using a small 22% relative
d
e
efficiency detector. Note v
r
e
s
the almost 50% loss of b
O
area for peak 244 keV at f
o

o
. i
close distances t
a
R
0.5
0 5 10 15
Source - Detector Distance (cm)
Simple Coincidence Theory
With no coincidence, the rate of pulses for peak
(cid:1)
g e
1 is n = Ap1 1
Co-60
where A = activity, p1 = emission probability of
g e
1, and 1 = peak efficiency at E1
g
With coincidence, the rate of pulses in peak 1 is
(cid:1)
e e e
n* = Ap1 1-Ap1 1 t2
e
where t2 is the probability of observing a count
g
1
g
due to 2 emitted at energy E2 anywhere in the
spectrum, i.e. the total efficiency at energy E2.
e
The correction is C = n/n* = 1/(1- t2) g
(cid:1) 2
Need to calibrate for total efficiency, or a
(cid:1)
Ni-60
combination of peak efficiency and peak-to-total
efficiency, which can be used to calculate the
total efficiency.
Total efficiency vs. Peak efficiency
1,000
• Total efficiency: ratio of all
Peak efficiency
counts in the spectrum to
500
the gamma rays emitted
by the source
300
Total efficiency
• Peak efficiency: ratio of all
200
counts in the photopeak
(at the emission energy) to
the gamma rays emitted 100
by the source.
• P/T ratio is the Peak 50
Efficiency divided by the
30
Total Efficiency.
20
0 100 200 300 400 500 600
Peak-to-Total Calibration
• The inherent P/T-ratio
– for all practical purposes, does not depend on
the source-to-detector distance
– it has been shown that for HPGe detectors up
to about 80% relative efficiency the use of the
intrinsic P/T-calibration during the integration
of the coincidence correction factor produces
sufficiently accurate results.
P/T ratio is essentially independent of sample
position relative to the detector
1 2
Mapping of P/T-ratio around detector
(no separation)
-0.8
3
Model: Constant
-1.0
Chi^2 = 0.00103
a(cid:1)-1.224
4 -1.2
-1.4
)
T
5 P/-1.6
(
n
l Zn
-1.8 Model: Constant
Mn
Chi^2 = 0.00303
6
a(cid:1)-1.614
-2.0
-2.2
7
1 2 3 4 5 6 7
Position of point around end cup
Empirical Peak-to-Total Curve
-0,2
-0,4
-0,6
-0,8
)
n
L -1,0
(

o
i
t -1,2
a
R

T -1,4
/
P
20% Canberra, 1997
-1,6
18% ORTEC, 1995
-1,8
20% Canberra, 2000
-2,0
100 1000
Energy, keV
Cascade Summing effects
Efficiency calibration is also
effected
Br-82, Sand Matrix, 15% detector
1.15
Corrected Activity
1.1
y
t
i Uncorrected Activity
v
i
t
c1.05
A

t
c
e
r 1
r
o
C

o
t0.95

o
i
t
a
R
0.9
0.85
0 500 1000 1500 2000
Energy (keV)