9.2 Geometrical Efficiency
151
counted, self-absorption remains constant as long as densities of the liquids are not
changed.
9.2.2 Backscattering
An important manifestation of the scattering of β-particle is the increase in count
rate obtained when a source is mounted on a solid backing material. The additional
count rate is due to β-particles which are scattered toward the counter through an
angle of 180
◦ . This effect can be explained by measuring the activity of a radioactive
sample kept over a planchet with an end-window G.M. counter. The sample is kept
near the window of the counter and beneath the planchet, a shelve is made to insert
some metal sheet. First, we measure the activity of the sample. And then we measure
the activity of the sample by inserting a thin aluminum sheet into the shelve. This
process is repeated by inserting an aluminum sheet of increasing thickness and the
corresponding increase in the activity is measured. One would expect that since
we have not changed the activity of the sample, or its position from the window,
the count rate in all these measurements should approximately be the same. On the
contrary, activity keeps on increasing exponentially as we increase the thickness of
the aluminum sheet, reaching a maximum value for some particular thickness of
aluminum. Any further increase in the thickness, does not lead to an increase in the
activity. The nature of the graph is similar to one represented in Fig. 9.2.
Why should activity increase when nothing has been done to the radioactive sample nor more radioactive material has been added? The radioactive sample emits
the radiation in all direction (i.e., in 4π direction). Those radiations moving toward
the window are recorded in a normal fashion (i.e., within 180
◦ ). But those radiations
which are emitted at angle 270
◦ or near about (i.e., > 180
◦
< 360
◦ ), are reflected
toward the window by the aluminum sheet kept beneath the planchet (Fig. 9.3). As a
result of this reflection, the activity recorded in the counter increases. As we increase
the thickness of the aluminum sheet, more and more radiations are reflected toward
the window. But this process of reflection comes to a maximum value when further
increase in thickness does not allow radiations to get reflected, but some of them
get absorbed by the aluminum sheet, hence are not in a position to reach the window of the counter. Thus, a saturation value reaches after a certain thickness of the
aluminum sheet. This phenomenon is known as backscattering. Theoretically, the
limiting value for increase in count rate is reached when the material is thick enough
to absorb all backscattered radiations from the deepest layer and are unable to penetrate the absorber to reach the window of the counter. The backscattering phenomena
increases with an atomic number of the element used. Hence, to minimize the errors
caused due to this effect, the source tray (planchet) is made of a material with a low
atomic number. In any comparative work, if the same source tray and the source is
used, this effect can be neglected.
151
counted, self-absorption remains constant as long as densities of the liquids are not
changed.
9.2.2 Backscattering
An important manifestation of the scattering of β-particle is the increase in count
rate obtained when a source is mounted on a solid backing material. The additional
count rate is due to β-particles which are scattered toward the counter through an
angle of 180
◦ . This effect can be explained by measuring the activity of a radioactive
sample kept over a planchet with an end-window G.M. counter. The sample is kept
near the window of the counter and beneath the planchet, a shelve is made to insert
some metal sheet. First, we measure the activity of the sample. And then we measure
the activity of the sample by inserting a thin aluminum sheet into the shelve. This
process is repeated by inserting an aluminum sheet of increasing thickness and the
corresponding increase in the activity is measured. One would expect that since
we have not changed the activity of the sample, or its position from the window,
the count rate in all these measurements should approximately be the same. On the
contrary, activity keeps on increasing exponentially as we increase the thickness of
the aluminum sheet, reaching a maximum value for some particular thickness of
aluminum. Any further increase in the thickness, does not lead to an increase in the
activity. The nature of the graph is similar to one represented in Fig. 9.2.
Why should activity increase when nothing has been done to the radioactive sample nor more radioactive material has been added? The radioactive sample emits
the radiation in all direction (i.e., in 4π direction). Those radiations moving toward
the window are recorded in a normal fashion (i.e., within 180
◦ ). But those radiations
which are emitted at angle 270
◦ or near about (i.e., > 180
◦
< 360
◦ ), are reflected
toward the window by the aluminum sheet kept beneath the planchet (Fig. 9.3). As a
result of this reflection, the activity recorded in the counter increases. As we increase
the thickness of the aluminum sheet, more and more radiations are reflected toward
the window. But this process of reflection comes to a maximum value when further
increase in thickness does not allow radiations to get reflected, but some of them
get absorbed by the aluminum sheet, hence are not in a position to reach the window of the counter. Thus, a saturation value reaches after a certain thickness of the
aluminum sheet. This phenomenon is known as backscattering. Theoretically, the
limiting value for increase in count rate is reached when the material is thick enough
to absorb all backscattered radiations from the deepest layer and are unable to penetrate the absorber to reach the window of the counter. The backscattering phenomena
increases with an atomic number of the element used. Hence, to minimize the errors
caused due to this effect, the source tray (planchet) is made of a material with a low
atomic number. In any comparative work, if the same source tray and the source is
used, this effect can be neglected.