Measuring build up factor and attenuation constant of
different materials using NaI(Tl)
Submitted to:
Dr. Asloob Ahmad Mudassar
Submitted by:
Yasir Ali
M.Phi. Physics DPAM
PIEAS
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Measuring Buildup Factor-Physics-Lab Report, Exercises of Physics
This is lab report for Physics course. It was submitted to Dr. Urmila Bhansi at All India Institute of Medical Sciences. It includes: Measuring, Build, Factor, Attenuation, Constant, Different, Materials, Sodium, Scattered, Radiation
Typology: Exercises
2011/2012
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Measuring build up factor and attenuation constant of
different materials using NaI(Tl)
Submitted to:
Dr. Asloob Ahmad Mudassar
Submitted by:
Yasir Ali
M.Phi. Physics DPAM
PIEAS
The purpose of the experiment is to determine buildup factor for given source. The experiment
involves firing a narrow beam of gamma-rays at a material and measuring how much of the
radiation gets through. We vary the type of absorbing material as well as its thickness and
density by interposing iron, aluminum, and copper absorbers of different thicknesses t between
the source and the detector. Buildup factor is determined by using two geometries, first the
counts are taken for good and bad geometry and then buildup factor is determined from it.
Another method used for measuring build up factor is determining buildup factor by using multi
channel analyzer.
Build up factor:- The factor by which the total value of the quantity being assessed at the point
of interest exceeds the value associated with only primary radiation. The total value includes
secondary radiations especially scattered radiation.
The gamma-ray buildup factors for point isotropic sources in infinite homogeneous media have
been widely used in gamma-ray shielding calculations. Considerable effort has been devoted
to developing methods of calculating buildup factors taking into account multiple scattering
of gamma-ray. The data set of gamma-ray buildup factors was first developed by Goldstein
theoretical value by some factor called build up factor. This increase in intensity of gamma rays
at observation point is due to many effects one of which is scattering of gamma rays from some
material. Theoretical value by some factor called build up factor. This increase in intensity of
gamma rays at observation point is due to many effects one of which is scattering of gamma rays
from some material.
Buildup factors vary with a number of parameters such as the distance of penetration through
the attenuating medium; the geometric configuration of the attenuating medium, source and
detector position; the composition of the medium; the detector response function; and the energy
and direction of emission of the source photons, ideally taken to be monoenergetic and
isotropic.
Why Build Up Factor Is Needed:- when radiation is incident on some attenuator material then
beside some other process also take place. The collision processes depend very much on the type
of particles involved in the collision. Heavily ionizing particles such as, alpha particles or
protons are very easily stopped by a small amount of material because they leave a dense trail of
ions. They are not generally removed by a single collision but slowed with energy going into the
ionizing process. On the other hand, electrons scatter off other electrons and in this process, lose
energy and produce a gamma. Subsequently, the gamma may react with another electron to
produce an electron and gamma. This process is called a gamma cascade which is complicated
to calculate.
Due to these many processes radiation passed through attenuator is not only given by attenuation
constant but also by another constant called build up factor. This build up factor has a number of
uses in radiation shielding.
Attenuation:-. When radiation falls on some material then some of radiation is blocked by
material by some processes like stopping or blocking by its nucleus. This property is used in
radiation shielding of material. Material having higher cross section of radiation stopping is used
in radiation shielding.
electron from inner shells of atom producing a vacancy there. This vacancy is filled by some
higher orbit electron which emits its energy as x-rays. This process also has some contribution to
attenuation.
Compton scattering:-. When radiation strikes some material, then photons of that radiation with
energy greater than binding energy of electrons of that material’s atom, then electron interact
with atom in such a way that they are scattered of electrons and giving some of its energy to
electrons and receiving a shift in its energy. This process contributes to attenuation.
Pair production:-. Photons with energy greater than 1.02MeV and pass near nucleus are
blocked by another process known as pair production. In this process photon of suitable energy
are converted into electron-positron pair spontaneously and photon disappears. Electron and
positron may recombine by process of annihilation so that photons of same energy are
reproduced, but this time they do not have direction same as original beam so they are scattered
and so this process contribute to attenuation.
In above discussion we saw that photons of incident radiation are stopped and scattered by
different processes. These scattered photons may reach to detector causing an increase in
intensity recorded by detector. So above formula with single attenuation coefficient μ is not
enough to calculate radiation reached after passing through shield. That formula is modified as
Where b is build up factor measured experimentally. It depends on energy of radiation and also
thickness of shield.
Build up factor is measured by following method of good geometry counts and bad geometry
counts.
Good Geometry:-. In such an arrangement only those photons are allowed to reach the detector
which suffers no collision with the shield or the photons which suffer Compton scattering with
the shield are not counted.
Bad Geometry:-. It is an arrangement of source, shield and the detector in which scattered
photons in the shield are also able to reach the detector. Bad geometry counts are always greater
than good geometry counts.
NaI(Ti) detector:-. Sodium iodide is very important detector due to relatively high atomic
number (Z = 53) of its iodine constituent ensures that photoelectric absorption is an important
process and detection efficiency. In Sodium iodide radiation interact with detector where charges
are produced and through the transmission of these charges an electronic signal is produced
using electronics setup.
Fig.2 bad geometery
Bad geometry does not require collimators so that scattered photons can reach the detector.
In good geometry collimators are needed because it minimizes the scattered photons to reach the
detector. These geometries can be mad using figures.
Apparatus Of Experiment:
1.Gamma rays source Cs-
2. Shielding material plates like Copper ,Aluminum ,Iron.
3. Collimators
4. Preamplifier.
5. Amplifier
6. Single Channel analyzer
7. Power supply
Taking ln of both sides we get
So measuring slope of line given by above equation give attenuation constant.
By this formula we can calculate attenuation constant. Similarly build up factor is measured by
Or
Ibg= bad geometry intensity.
Igg = good geometry intensity
1. Good geometery:-. Let use different materials
- Iron: Tabulated Data: Plate No X1 X2 X3 X=(X1+X2+X3)/ Count rate Count rate2 Average ln(Average) 0 0 0 0 0 162346 162330 162338 11. 4 6.8 7.225 7.075 7.03333333 114775 115232 115003.5 11. 2 6.37 7.075 6.4 6.61666667 81395 81650 81522.5 11. 13 6.25 6.25 6.275 6.25833333 57664 57827 57745.5 10. 10 6.27 6.25 6.35 6.29166667 58244 41304 49774 10. 9 6.37 6.175 6.45 6.33333333 29613 30220 29916.5 10.
FIG. 4 for attenuation constt of Iron
The value of attenuation coefficient is 0.3404 mm-
2. For Aluminum:
Tabulated Data:
Plate No X1 X2 X3 X=(X1+X2+X3)/ Count rate Count rate2 Average ln(Average) 0 0 0 0 0 162346 162330 162338 11. 24 6.3 6.25 6.125 6.225 141789 141201 141495 11. 27 6.175 6.075 6.25 6.16666667 125131 124378 124754.5 11. 25 6.15 6.175 6.25 6.19166667 111015 108782 109898.5 11. 30 6.175 6.15 6.225 6.18333333 97344 98014 97679 11. 20 6.3 6.343 6.2 6.281 86740 87000 86870 11.
The value of attenuation coefficient is 0.766 mm-
Bad Geometry:-.
The so called data obtained for a bad geometry configuration is tabulated under the name of each
material.
1. For Copper:
Tabulated Data:
Plate No X1 X2 X3 X=(X1+X2+X3)/ Count rate Count rate2 Average 0 0 0 0 0 1101460 1102080 1101770 8 13.1 12.45 12.4 12.65 682940 684767 683853. 4 12.45 13 12.45 12.6333333 373868 374364 374116 1 12.375 12.3 13.075 12.5833333 196033 194720 195376. 6 13.05 12.42 12.45 12.64 100321 100362 100341. 5 12.42 12.37 12.4 12.3966667 51516 52461 51988.
2. Aluminum:
Tabulated Data:
Plate No X1 X2 X3 X=(X1+X2+X3)/ Count rate Count rate2 Average 0 0 0 0 0 1101460 1102080 1101770 24 6.3 6.25 6.125 6.225 1032480 1033548 1033014 27 6.175 6.075 6.25 6.16666667 967720 966397 967058. 25 6.15 6.175 6.25 6.19166667 902247 903017 902632 30 6.175 6.15 6.225 6.18333333 839061 838876 838968. 20 6.3 6.343 6.2 6.281 775796 776260 776028
3. Iron: Tabulated Data
Plate No X1 X2 X3 X=(X1+X2+X3)/ Count rate Count rate2 Average 0 0 0 0 0 1101460 1102080 1101770 4 6.8 7.225 7.075 7.03333333 907305 907388 907346. 2 6.375 7.075 6.4 6.61666667 725568 726078 725823 13 6.25 6.25 6.275 6.25833333 566222 566544 566383
Since attenuation coefficient is already calculated so we do not need to calculate it again.
Build up factor measurement:-.
The buildup factor corresponding to different value of thickness for the three materials is
tabulated and is graphically represented in the following paragraphs.
Buildup Factor by SCA
a) Iron
Graphical Representation:
No. of Plates Accumulative Thickness Good Geometry Bad Geometry Buildup Factor(Ibg/Igg) 0 0 162338 1101770 6. 1 7.033333 115003.5 907346.5 7. 2 13.65 81522.5 725823 8. 3 19.90833 57745.5 566383 9. 4 26.2 49774 437980 10. 5 32.53333 29916.5 333587 11.
Graphical Representation:
Fig. 8, build factor versus thickness
Buildup Factor by MCA
a) Iron
Plate No. Accumulative
Thickness Gross Counts(Cg)
Photo Peak Counts(Cp) Buildup
Factor(Cg/Cp)
b) Aluminum
Plate No. Accumulative
Thickness Gross Counts(Cg)
Photo Peak Counts(Cp) Buildup
Factor(Cg/Cp)
Dependency of Attenuation Coefficient on Atomic Number of Material:-. We can compare attenuation coefficient of different material. If we consider atomic number of different material and their attenuation coefficient, we conclude that it is proportional to atomic number.
Data:
Material Z μ (mm-^1 ) Aluminum 13 0. Iron 26 0. Copper 29 0.
Graphical Representation:
Fig. 9. Attenuation coefficient versus atomic number
Conclusion:-. From all above data we can conclude that attenuation coefficient is dependent