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Stacking Samples:
General Considerations
Stacking Samples: The Paradoxical Effect of Shielding
Samples on a Table (or The Advantages of Foam Sample Holders)
When planning beam
time requests, an important consideration is how many samples can be exposed
at the same time. The large beam spot with uniform illumination (about
20 x 20 cm2) allows for as many as four T75 flasks or six T25
flasks to be contained within the beam center. Under certain
conditions it is possible to stack multiple samples along the beam
direction. The effect of stacked samples can either increase the dose
or decrease the dose depending on the heavy ion being used, the beam energy,
and other considerations. The details of dose delivery need careful consideration
before sample stacking is utilized.
In general,
for heavy
ions like Fe-56, fragmentation of the primary ion in the upstream samples
results in a lower dose being delivered to the downstream samples. Of
course the same consideration applies to thick samples that are not stacked.
There are two effects at work in the case of heavy ions. As the ion
passes through the sample, it slows down which increases LET (in general).
Heavy ions can fragment in the sample, with the resulting lower-Z fragments
depositing lower LET (in general) than the primary high-Z ion. Under
certain conditions it is possible for slow fragments to deposit greater LET
than the primary ion would have. In cases where thick samples or
multiple stacks of samples are demanded, detailed calculations are required
to fully characterize the dose delivered.
Figure 1: Measured effects of stacking 3 T25 flasks in a 1000 MeV
proton beam. Top shows flasks without any medium, while the bottom
shows the effect of filled flasks.
For low-Z ions and
protons in particular, stacking samples can have the effect of increasing
the dose delivered to downstream samples due to target fragmentation and
secondary particles in the beam produced by interactions of the beam
particles with the sample. As an example, 1000 MeV protons incident on
an aluminum target of thickness 20 g/cm2 produced a dose profile
shown in Figure 2 where the measurement is compared with the results of a
simulation (PhiTs program). The dose delivered is more than 50%
greater at the exit of the target than in the absence of the target.
An interesting observation is that back-scattered particles elevate the dose
at the upstream side of the target as well, making the dose 10% higher
immediately in front of the shield.
Figure 2 Comparison of measurement and simulation of the dose delivered
by a 1000 MeV proton beam in the neighborhood of an aluminum shield of
thickness 20 g/cm2. A detailed analysis of the simulation
showed that the dose increase is due primarily to secondary protons knocked
out of the aluminum shield.
At higher energies the effect increases, as illustrated with the
measurement of 2500 MeV
protons below. The dose rate immediately behind a shield of 15 g/cm2
aluminum is 61% higher than the nominal rate. For a shield of 50 g/cm2
thickness, the rate increase is 130%.
Figure 3 shows the increase in dose delivered by a 2500 MeV proton beam
immediately behind an aluminum shield, and 10 cm downstream of the shield.
The effect of secondary particles and scattered
particles applies also to samples placed on top of or close to a table or
other massive surface. Measurements of dose near the surface of an
aluminum table top that is in the beam show the effect. Although the
magnitude of the effect is small, it should be kept in mind when deciding on
sample holders and placement.
Last Modified: February 1, 2008
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