LANL Target Design
MARS Calculations
Randy Johnson
April 12, 2000
Geometry Simulated
The geometry simulated in the MARS calculation was for a 65 cm long,
5 mm radius Be target surrounded by two concentric cylinders of Al or Be.
The inner cylinder was .8 mm thick with an outer radius of 1.27 cm. The
outer cylinder was .8 mm thick with an outer radius of 1.59 cm. The material
between the cylinders was taken to be air. Nothing was simulated
outside of the outer cylinder or downstream of the target. No fins or
cylinder supports were simulated in the target region.
Target Activation
The Be target activation (1.47 stars/inc. proton) is very similar
to the activation presented at the
November collaboration meeting (1.4 stars/inc. proton). Please look at
that web page
for a sample activation calculation. In equilbrium, this represents 49.7 Ci
of Be7. At 2', this represents a dose of 5.45 Rad/hr of .5 MeV photons.
Plots of the star density for the activations are given in
Figure 1
for the Al jacket and
Figure 2 for the Be jacket.
Cylinder Activation - Al Case
The total number of stars produced in the cooling cylinders
was .0349/inc. proton and .0350/inc. proton
for the inner Al cylinder and the outer cylinder respectively. Thus, under
equilibrium conditions, this would represent 1.2 Ci of Na22 and 2.4 Ci of
Be7 (for a total of 52.1 Ci Be7). This represents a total dose of
5.2 Rad/hr at 2'
(including the .5 MeV photons from the Na22) in addition to the target
for a total of 10.7 Rad/hr at 2'. 8.3 cm of iron will shield this to
100 mRad/hr at 2'. Of the 100 mRad/hr,
66 mRad/hr is coming from the 1.27 MeV Na22
photons while 34 mRad/hr comes from the .5 MeV photons from both the Na22
and the Be7.
Cylinder Activation - Be Case
The total number of stars produced was .029/inc. proton and .030/inc. proton
in the inner and outer Be cylinders respectively. This represents an
additional 2.0 Ci of Be7. Combined with the target, there would be a total
of 51.7 Ci which gives a combined total dose of 5.7 Rad/hr at 2 ft.
To reduce this dose
to .1 Rad/hr would require 4 absorption lengths or 6 cm of iron.
All of these numbers are for immediately after shutdown. The relatively
short half-life of Be7 (54.5 days)
allows a pure Be9 target to cool in a reasonable
length of time and therefore allows a thinner coffin.
Air Activation
The air activation is .127 stars/inc proton with most of this activation
coming from the region right around the target.
Warning to MARS users
The cooling tubes of the target are so thin that you have to reduce the
MARS step size to get an accurate picture of the activation. I inadvertently
kept the step size a 1 cm when I first ran the jobs. The comparison of the
star density for a 1 cm step size and the .1 mm step size (which was used for
all of the above results) is shown in
figure 3.
The big step greatly underestimates
the activation.