1. Single trigger, no target
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.
2001 T. Baumann
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].
MoNA
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.
Neutron detection
Helium 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).
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 ns/ch.)
Key to graphs:
Raw position graph.
Position with cut from veto bar TOF.
Position with cut From ΔE TOF.
Position with cuts from veto TOF and ΔE TOF.
Position with cuts from ΔE TOF, ΔE energy and veto energy 4 2 1 3 5
Single trigger, no target 4 2 1 3 5
Singles trigger, without a target
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.
5.6mm Carbon target 4 2 1 3 5
No target, Carbon & Boron tuned by A1900
We identified the large peak in the above graph as carbon and boron. This run was done to verify our identification.
The single-trigger plot has no neutrons left (5) when all cuts are applied (No target to produce neutrons).
The 5.6 mm target was used since it has a higher cross-section than the 3.3 mm. The thicker target was expected to give us a larger number of neutron events and 6He counts. 5
3.3mm Carbon target 4 2 1 3 5
A 36Ar 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.
The Experiment
8Li – p+  7He
7He  6He + n
3.3mm Carbon target
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.
6He
8Li
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.
5.6mm Carbon target
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.
6He
8Li
To receive the 6He 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-cm-thick 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.
ΔE-E arrangement
Experimental setup
64.5 cm
470 cm
Thin scintilator (start) detector
MoNA Bars
BaF2 detector
ΔE detector
Veto paddles
Window
Experimental Setup Diagram
Not to scale
References
[1] T. Baumann et al., Nucl. Instr and Meth. B, 192 (2002)
[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.
Carbon Target
8Li Beam