783620H / 1013 6. ANALYSIS METHODS
table and the program uses a linear interpolation between the table values and linear extrapolation outside the table values. This underestimates the correction below the lowest energy because the attenuation is logarithmic in form.
Table 10. Absorption.
Energy
Without With Ratio
123.14 63.23 40.71
1.553
248.04 7.80 5.36
1.455
591.74 3.31
723.3 11.50
873.20 5.75
1004.76 7.71
1274.45 10.29
2.68
8.79
4.69
6.25
8.65
1.235
1.308
1.226
1.234
1.190
Log(ratio)
0.440
0.375
0.211
0.269
0.204
0.210
0.174
Table 11. Results — Measured Correction.
Nuclide No Absorber
154
Eu
155
Eu
125
Sb
12750 Bq
8020
2900
Uncorrected
Absorber
10900 Bq
4813
2190
Corrected
Absorber
12750 Bq
7621
2890
6.10.6.4. Example — Table Values
The same spectrum example can be used to show the calculated coefficients. Using the mass attenuation coefficients table
33
for silicon and oxygen, the linear attenuation coefficients can be calculated for the sand absorber. Any other materials in the sand are ignored in this case, but might not be in the general case. Table 12 shows the mass attenuation coefficients for oxygen, silicon and sand (SiO
2
).
The sand value can be calculated using the mass ratios. In this case it is also given in the handbook tables. The density of the sand was measured to be 1.67 g/cm
3
. The fifth column in the
33
“Radiological Health Handbook,” January 1970, U.S. Dept. of Health, Education and Welfare.
289
GammaVision
®
v7 (A66-BW) 783620H / 1013
table is the coefficient in column 4 multiplied by the density. These numbers are entered into the
SOR table. This is the linear attenuation factor in 1/cm.
In the analysis, the absorption factor is entered as 1.6, because this is the thickness of the sand in cm. The result of the analysis using this table is shown in Table 13. Note that the calculated correction is more accurate below 100 keV than the measured correction so
155
Eu is more accurately corrected. Figure 252 shows the two correction files. The vertical scales have been adjusted to account for the difference in the input factor of 1.6.
Energy
30
40
50
60
80
100
150
200
300
400
500
600
800
1000
1500
2000
Oxygen
.372
.257
.213
.191
.168
.156
.136
.124
.107
.0957
.0873
.0808
.0708
.0637
.0518
.0446
Table 12. Calculated Coefficients.
mu/rho
Silicon
1.41
.696
.437
.322
.224
.184
.145
.128
.108
.0962
.0875
.0808
.0707
.0635
.0518
.0448
.108
.0959
.0874
.0808
.0707
.0636
.0518
.0447
Sand
.859
.463
.318
.252
.194
.169
.140
.126
mu
Sand
1.43
.773
.531
.421
.324
.282
.234
.210
.180
.160
.146
.135
.118
.106
.0865
.0746
Nuclide
154
Eu
155
Eu
125
Sb
Table 13. Results — Calculated Correction.
No absorber
12750 Bq
8020
2900
Uncorrected absorber
10900 Bq
4813
2190
Corrected absorber
12450 Bq
7965
2776
290
