Aluminum Activation
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Aluminum Activation
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Neutron activation of aluminum may occur by several neutron capture reactions. Four such re
t; ns are
described here: 27A1 + n = 28A1, 27Al(n,a )24Na, 27Al(n,2n)26Al and 27Al(n,p)27Mg. The radioactive
nuclei 28Al, 24Na, and 27Mg, which are produced via the 27A1 + n = 28A1, ‘27Al(LCt)24Na and
27Al(n,p)27Mg neutron reactions, beta decay to excited states of 28Si, 24Mg and 27A1 respectively.
excited states then emit gamma rays as the nuclei de-excite to their respective ground states.
These
Neutron energies Etie~al -.025 eV E~loW-1 keV Efast - 10OkeV-1 OMeV
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12/8/1999 11:03 AM
DISCLAIMER
This repoti was prepared as an account of work sponsored
by an agency of the United States Government.
Neither
the United States Government nor any agency thereof, nor
any of their employees, make any warranty, express or
implied, or assumes any legal liability or responsibility for
the accuracy,
completeness,
or usefulness
of any
information, apparatus, product, or process disclosed, or
represents that its use would not infringe privately owned
Reference
herein “to any specific commercial
rights.
product, process, or service by trade name, trademark,
manufacturer, or otherwise does not necessarily constitute
or imply its endorsement, recommendation, or favoring by
the United States Government or any agency thereof. The
views and opinions of authors expressed herein do not
necessarily state or reflect those of the United States
Government or any agency thereof.
DISCLAIMER
Portions of this document may be illegible
in electronic image products.
Images are
produced from the best avaiiable original
document.
http:llphysics.geneseo .eduhmke/27alncap ,htm
27A1n capture
Neutron Capture
Neutron activation of 27A1 forming 28A1 which beta decays to 28Si* which subsequently de-excites via
gamma emission to 28Si is shown below schematically. Note: this reaction has a large thermal neutron
;apture cross section of- 12 barns.
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12/8/1999 11:15 AM
http://physics.geneseo.eduhmke/al27na.htm
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The 27Al(n,a )24Na reaction and decay scheme are shown below. The 24Na beta decay feeds into the
second excited state of 24Mg 99.944°/0 of the time. The transition from the second to the first excited
state of 24Mg emits a 2.754 MeV gamma ray 99.944°/0 of the time, Subsequently, the first excited state
de-excites 100% of the time to the ground state, emitting a 1.368 MeV gma
ray making this transition
a good indicator of the measure of activation of the aluminum sample.
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27A1n p
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z7Al(n,p)z7Mg
Shown below is the 27Al(n,p)27Mg reaction and decay scheme. In this reaction a proton is produced
promptly after the neutron capture leaving 27Mg. The beta decay of 27Mg feeds the first and second
excited states of 2TAI. These states de-excite and emit 0.843 and 1.014 MeV gamma rays with branching
fractions of 71 % and 29 % respectively.
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12/8/1999 11:19AM
27A1n 2n
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z7Al(n,2n)z6A1
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Gamma Ray Spectra
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Gamma Ray Spectra
An aluminum sample was bombarded with mono energetic 14.1 MeV neutrons. The resulting gamma
ray spectrum was obtained by a High Purity Germanium (HPGe) detector after the activation and is
shown below.
Gamma ray spectrum
2000
1500
843.7 keV
~
~
a
v
~~
\
1000
k
500
1014 ke’d
1368 keh’
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0
0
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250
500
750
I
1000
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1250
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1500
1750
12/8/1999 11:21 AM
http://physics.geneseo .edulnuke/g~_ray_spectra.htrn
Gamma Ray Spectra
2000
Gamma ray spectrum
expanded frmm 800 to 1400 keV
1500
500
I
0
800
900
1000
1100
1200
1300
energy in keV
Note that there is no 1779 keV peak produced by the 27A1 + n = 28A1 capture reaction. However, peaks
from the 27Al(n,a )24Na and 27Al(n,p)27Mg reactions are easily seen in the HPGe detector spectrum.
In general, the number the gamma rays emitted by the activated aluminum sample is determined by
several factors. Some of the factors are fixed by nature, such as the reaction, the neutron capture cross
section, the half life, the efficiency of the detector, and the branching ratio of the gamma ray transitions
in the decay products. Others maybe changed in the experiment, such as the radial position of the
sample from the neutron source, the mass of the activation sample and the counting time. The neutron
capture threshold energy is another important factor. In the 2TAl(n,cx)24Na and 27Al(n,p)27Mg reactions
thermal or low enemv neutrons do not activate the sa.nmle. While in the 27AI + n = 28A1 ca~ture reaction
the cross section is~&ge(12 barns!) for thermal neutro&. Hence the peak is seen in the Nfi detector
spectrum and not the ~Ge detector spectrum. Shown below are plo& of the neutron capture cross
sections for the 27Al(n,u )24Na and 27Al(n,p)27Mg reactions.
The NaI detector spectrum shown below was produced using a NaI detector measuring gamma rays
emitted by an activated aluminum sample (note the reduced resolution of the NaI detector compared to
the HPGe detector).
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http://physics.geneseo.eduhmke/gamma_ray_spectra.htm
Gamma Ray Spectra
gamma ray spectrum
Nal detector
4000
3000
/
511 kev
IX
d
843.7 keV
..
1779 keV
1000
x’-”
0
0
t
I
tI
,
I
500
1000
1500
1
2000
I
2500
I
3000
energy (keV)
neutrons produced born a PuBe (plutonium-beryllium) source. (This is unlike the monoenergetic 14.1
MeV neutron activation resulting in the HPGe detector spectrum). The PuBe source produces neutrons
ranging in energy fi-om .025eV (thermal neutrons) to 11 MeV neutrons (shown on the next page). The
PuBe source was immersed in water which thermalized the higher energy neutrons, and further increased
the likelihood of the 2TA1+ n = 28A1 capture reaction. This occurs because the 2TA1+ n = ZSA1reaction
has a large thermal neutron cross section. Thus the 1779 keV peak produced by this capture reaction is
seen in the NaI detector spectrum and not in the HPGe detector spectrum.
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http://physics.geneseo ,edu/nuke/gamma_ray_spectra.htm
Gamma Ray Spectra
.
.
.
PuBe Neutrcm Spectrum
8
6
4
2
t
I
0.0
2.(I
4.0
6.0
8.0
10.0
12.0
Energy
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DT neutron yield measurements using neutron activation of alurninWp://physics. geneseo.edu/backgro..._ neutronflield_measurements_us.htm
.
‘r
DT neutron yield measurements
using neutron activation of aluminum.
Stephen Padalino, Heather Oliver and Joel Nyquist
LLE Collaborators:
Vladimir Smalyuk and Nancy Rogers
The primary neutron yield, for more than 40 ICF shots, was determined by measuring the activ tyofall
aluminum sample activated by DT neutrons. The reuslts were compared to the existing copper ,ctivation
method. The radioactive nuclei 24Na and 27Mg, which were produced via the 27Al(n,u )24Na and
2TAl(n,p)27Mg direct reactions, beta decay to excited states of 24Mg and 27A1 respectively. These
excited states then emitted gamma rays as the nuclei de-excite to their respective ground states. The
gamma rays are detected and counted. From their numbers the neutron yield is determined.
The DT neutron yield, Yn, as measured by counting gamma rays in the HPGe detector is given by the
equation.
Where Q is the solid angle of the aluminum sample, n is the number density of aluminum, o is the
nuclear cross section for 14.1 MeV neutrons and } is the thickness of the aluminum sample. ~ (E) is the
efficiency of the HPGe detector at the gamma ray energy of interest and L is the decay constant for 24Na
or 27Mg. Tw is the “wait time” between activation of the sample and the time when counting begins and
tc is the gamma ray counting time. NY is the number of gamma rays detected by the HPGe and R(E) is
the gamma ray re-absorption coefficient for aluminum.
Three gamma ray peaks are prominent in the gamma ray energy spectrum. They are produced by two
nuclear reactions, which lead to the decay of 24Mg and Z7AI. The decay scheme for 24Mg is shown
below.
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As shown above the 24Na beta decay feeds into the second excited state of 24Mg 99.944% of the time.
The transition ilom the second to the fust excited state of 24Mg emits a 2.754 MeV gamma ray 99.944?40
of the time. Subsequently, the first excited state de-excites 100°/0 of the time to the ground state, emitting
a 1.368 MeV gamma ray making this transition a good indicator of the measure of activation of the
aluminum sample. However the efficiency of the high purity germanium (HPGe) detector, used in these
measurements, has a low detection efficiency of 2.7 0/0at 1.368 MeV. The gamma ray detection
efficiency as a function of energy for the HPGe well detector was measured using a 9 line calibrated
gamma ray source at 10 well depths. The calibration curve at 1 cm from the bottom of the well is shown
below. The fill set of efficiency curves for all 10 depths are not shown here.
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... neutron_yield_measurements_us .htm
geneseo.ed~ackgro.
HPGe well detector efficiency@
1 cm well depth
40
‘1
‘k
10
‘--=%.
--’ G-. —
c1
0
1
t
200
400
I
600
1
--o-.
I
800
1000
-.+._
.. —=
I
I
1200
1400
__
1
1mm
-0
1
1800
2000
Energy in keV
At higher gamma ray energies the detection efficiency drops further. This makes it impractical to use
higher energy transitions, such as the 2.754 MeV gamma ray in 24Mg in counting measurements with
our detector. In contrast, the efficiency is relatively good near 0.7 MeV, approximately 7 O/O.However,
the gamma ray background is large and the signal to noise ratio is poor. Our project showed that the
gamma ray background at 0.843 MeV is low enough to make a good measurement with reasonable
counting statistics.
The production of the 0.843 MeV gamma is shown below in the level diagram for 27A1. The beta decay
of 27Mg feeds the first and second excited states of 27A1. These states de-excite and emit 0.843 and
1.014 MeV gamma rays with branching fractions of 71 YOand 29 % respectively.
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.
.
.htm
.
Shown in the next figure is a typical spectrum. The good signal to noise ratio for this gamma peak,
coupled with a reasonable detector efficiency, favorable branching fraction and the 9.5 minute half-life
made this the best choice for a neutron yield measurement. Due to the physical limitation of the
aluminum sample size and half-life of 27Mg, yields of less than 108 neutrons produced poor gamma
counting statistics for times greater than 45 minutes which is the maximum time allowed between
experimental events.
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DT neutron yield measurements using neutron activation of alurnin&ttp://physics. geneseo.edu/backgro... _neutron~ield_measurements_us
.htm
.
.
.
Gamma ray spectrum
2ilcio 7
1500!
843.7 keV
,
o
250
500
750
1000
11114keV
1250
1500
1750
2000
Gamma ray spectrum
expanded from 800 to 1400 kdl
1500
g
5 1000
a
u
500
0
-
800
900
1000
1100
1200
1300
1400
energy in keV
The sensitivity of the aluminum activation method was modified to cover a large dynamic neutron yield
range. Some of the factors are fixed by nature and the detector, such as the neutron capture cross section,
the half life, the efficiency of the HPGe detector, and the branching ratio of the gamma ray transitions in
the decay products. Others could be changed, such as the radial position, mass or type of the activation
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DT neutron yield measurements using neutron activation of ahunindrttp://physics.geneseo.edu/backgro... _neutron~ield_measurements_us
*
.htm
smple and the counting time. The neutron threshold energy is also important and must be high enough
to exclude thermal or low energy scattered neutrons. Shown below is a plot of the neutron reaction cross
section for the 27Al(n,x) reactions.
Rwmtkm Crass Sections
Note that the cross section for neutron capture in the 27Al(n,cx )24Na reaction at 14.1 MeV is 121.2
millibars. Compare this to 72.3 millibams for the 27Al(n,p)27Mg reaction. Furthermore the n,n’
reaction is prompt and does not play a role in our measurement time scale.
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geneseo.eduibackgro..._ neutronflield_measurements_us.htm
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..
E2EEEzl
Cross Section at 14.1 MeV
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0.123 barns
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0.073 barns
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0.004 barns
“ZEzEzl
k
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PEEl
0.433 barns
The aluminum activation experiment used the 843 KeV gamma ray counting measurement to determine
the neutron yield from DT reactions. These measurements were then compared to the yields measured
with the copper activation method. A comparison of aluminum method vs. the copper
. . method is shown
below for 40 Omega shots.
The measurements reveal a very strong correlation between the two methods. The absolute yield
produced by the aluminum method is consistent with the copper method by to 1?4.
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