Transcript Document

First Radioactive Beam Experiment With
the Modular Neutron Array MoNA
Mustafa Rajabali a, Melanie Evanger a, Ramsey Turner a, Bryan Luther a, Thomas Baumann c, Yao Lu b, Michael Thoennessen b,c, Erik Tryggestad c
a Concordia College, Moorhead, MN
b Michigan State University , East Lansing, MI
c National Superconducting Cyclotron Laboratory, East Lansing, MI
Abstract
μMoNA, a small version of MoNA consisting of 8 MoNA bars, was used to study the feasibility of detecting 7He which decays in flight into 6He and a neutron. A secondary beam of 8Li at 85 MeV/u bombarded a 5.6 mm and a 3.3 mm carbon target in two
different runs. States in 7He were populated via the single proton stripping reaction. The subsequent 6He fragments were detected at zero degrees with a ΔE-E detector arrangement consisting of a 1-cm-thick plastic scintillator and 10-cm-thick BaF2 detector. The
corresponding neutron from the 7He decay was detected with μMoNA, which was configured in two rows of four detectors above and below the BaF2 detector. From the energy of the 6He and the position and time-of-flight of the neutron, it should be possible to
kinematically reconstruct the populated states of 7He. The experimental setup and some preliminary data from the study is presented here.
MoNA
The Modular neutron array (MoNA) is a large-area neutron
detector to be located at the NSCL for the investigation of
neutron-rich nuclei [1,2].
MoNA will have a front face area of 2 m x 1.6 m and will
consist of 144 individual detector modules (see rendering at
left). Each independent module consists of a BC-408 plastic
scintillator bar, 2 m long and 10 x 10 cm2 in cross section, with
light guides and phototubes at each end [3].
Helium Detection
The plots below are 2D plots of the time of flight (vertical
axis) and energy (horizontal axis) of the ΔE detector. (Units
displayed are channel number. Time to channel conversion
is 0.089 ns/ch.)
Singles trigger, without a target
2001 T. Baumann
The neutron position on a module is found from the
timing difference between light arrival at the two phototubes.
Energy is deduced by the neutron time of flight from the start
detector to the individual module.
μMoNA (a small version of MoNA) consisting of eight
detector modules was used for neutron detection in this
experiment.
This run was taken
with the 8Li beam,
without a target. This
was to see were the
constituents of the
beam fell in the
spectrum.
We identified the large
peak in the above
graph as carbon and
boron. This run was
done to verify our
identification.
No target, Carbon & Boron tuned by A1900
The Experiment
A
primary beam on a beryllium target was used to
make a secondary 8Li rich beam which was bombarded onto a
carbon target under two different situations. One was with
the target being 5.6 mm thick and the second with the target
3.3 mm thick. (Impurities found in the beam included carbon
and boron, whose identification will be discussed later.)
On hitting the carbon target, the reaction fragment
with highest population is expected to be 7He which decays
instantaneously to 6He giving off a neutron.
6He comes off in a narrow cone while neutrons come
off at a significantly larger cone.
A 3.3 mm carbon target
was placed in the pod, the
large peak at ~channel 150
(energy-horizontal axis) is
8Li and the smaller peak at
~channel 20 is the 6He.
36Ar
6He
8Li
7He
–
p+

8Li
3.3mm Carbon target
7He
 6He + n
6He
8Li
Experimental setup
ΔE-E arrangement
To receive the
and for the identification of other
charged fragments or particles, a ΔE-E arrangement was
placed along the beam axis. The ΔE detector was a 1-cmthick plastic scintillator wrapped with black tape for light
tightness. It was placed in front of the E detector, which was
a 10-cm-thick cylindrical BaF2 crystal.
To accommodate the ΔE-E telescope and to measure
the neutrons, the eight MoNA bars were separated into two
layers of four bars. The layers were placed above and below
the ΔE-E arrangement so that a neutron coming off at an
angle from a decaying 7He (along beam axis) could be
detected by one of the four bars in either of the layers.
5.6mm Carbon target
6He
Another run was made this
time with a thicker target
(5.6 mm) to improve the
count rate of the 6He. As
we can see, The smaller
peak at ~channel 20 has
now slightly more
intensity.
Experimental Setup Diagram
ΔE detector
Veto paddles
Thin scintilator
(start) detector
8Li
MoNA Bars
BaF2 detector
470 cm
Conclusion
From the results of the experiment we were able to detect neutrons in
correlation with 6He. The energy resolution, however, was not good enough
to kinematically reconstruct the states of 7He.
We conclude therefore that it is feasible to observe the 8Li break up
into 7He by detecting the 6He and neutron from the 7He with the full MoNA
detector. MoNA’s large active area allows for a longer flight path hence better
energy resolution.
Members of the MoNA collaboration have proposed repeating the
experiment on a larger scale using the completed MoNA detector with large
veto paddles and an improved central ΔE-E telescope.
References
Window
Carbon Target
Neutron detection
The 1D plots below show our neutron detection on the MoNA bars. The
position of the particle deposition on the horizontal axis is channel number
( which corresponds to position along the bar) and the vertical axis has the
particle counts (in logarithmic scale).
Key to graphs:
1 Raw position graph.
Single trigger, no
2
target
Position with cut from
veto bar TOF.
4
3
3
2
Position with cut From
1
ΔE TOF.
4
Position with cuts from
5
veto TOF and ΔE
TOF.
5
5.6mm Carbon
Position with cuts from
target
ΔE TOF, ΔE energy
and veto energy
4
3
2
The single-trigger plot
1
has no neutrons left (5)
when all cuts are
5
applied (No target to
produce neutrons).
The 5.6 mm target
3.3mm Carbon
was used since it has a
target
higher cross-section
4
3
than the 3.3 mm. The
2
1
thicker target was
expected to give us a
larger number of
5
neutron events and 6He
counts.
Beam
64.5 cm
Not to scale
[1] T. Baumann et al., Nucl. Instr and Meth. B, 192 (2002) 339-344.
[2] B. Luther et al. Nucl. Instr and Meth. B, in press.
[3] P.J. Van Wylen et al., Poster 5P1.071 presented at this conference.
This work supported in part by
grants from the National Science
Foundation and the NSCL