The MoNA Project and Concordia College
The MoNA Project and Concordia College |
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MoNA (the Modular Neutron Array) is a detector array specialized to detect neutrons, the neutral particles in the core of atoms. It will be used in conjunction with high energy radioactive ion beams produced at the National Superconducting Cyclotron Laboratory (NSCL). MoNA will consist of 144 individual detector modules that will enable experimenters to determine the energy and momentum of neutrons created in the nuclear reactions that occur when the rare isotope beam strikes a target.
Concordia College is one of 10 colleges and universities that are collaborating on the construction and testing of the MoNA detector. Three Concordia students (Melanie Evanger, Mustafa Rajabali and Ramsey Turner) are working on the MoNA project under the direction of Dr. Bryan Luther. They are working at the NSCL during the summer of 2002.
The first job of the Concordia team was to assemble the 32 of the 144 detector modules to be used in MoNA.
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The 2 meter long plastic scintillator bars arrive from the manufacturer, Saint Gobain. Inc., in crates of 8 bars. Each bar has a plastic light guides and metal flanges attached to each end. The bars are wrapped in light-tight black plastic and tape. Each bar weighs about 20 kilograms.
The bars are removed from the crates and the end surfaces are carefully cleaned. The internal reflections that trap light inside the bar can be seen in the photograph on the above right.
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In order to convert the light pules from the scintillator material into useable electrical signals a photomulitiplier tube, magnetic shield and voltage divider must be coupled to the ends of the bar. Melanie Evanger prepares one of the phototubes for coupling to the bar ends in the photo to the left. An exploded diagram of the photoassembly components is shown on the right.
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On the left the Concordia team (Melanie Evanger, Mustafa Rajabali and Ramsey Turner) stand next to 8 completed detector modules. On the right is a close-up view of the detector phototube assemblies. The metal sleeve is the magnetic shield and the high voltage and signal connections can be seen at the base of the voltage divider.
Once the modules have been assembled they need to be tested and characterized. The linearity of timing and signal response needs to be checked and the attenuation length, timing resolution, and voltage/gain relationship needs to be determined.
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Ramsey Turner records readings taken with the 207-Bismuth radioactive source on the left. The Concordia team discusses results of some of the measurements on the right. The light blue electronics rack holding the equipment needed to make the measurements on the modules can be seen on the right.
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Some of the pulse height spectra obtained in testing are shown above. The spectrum on the left is from 137-Cesium while the spectrum on the right shows the 20.7 MeV cosmic-ray energy deposition peak in the bar.
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A time difference spectrum taken with cosmic rays shows the "length" of the detector module in time. The change in channel numbers corresponds a time for a light signal to pass from one end of the bar to the other (approximately 12 nanoseconds). |
By measuring the changes in signal strength on the individual phototubes as the source is moved down the bar an attenuation length can be determined. (Attenuation length is measure of how much the signal diminishes as it travels down the module.) Position of neutron events will be determined by differences in arrival time of the light pulses at the two ends of the module. The linearity of this technique can also be determined using sources.
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The graphs above shown sample data for attenuation length (left) and time-difference linearity (right) measurements.
Once the bars are measured and tested they are carefully stored in crates to await final assembly. Mustafa Rajabali is shown below preparing the crates and placing the modules inside. (And having way too much fun cutting up the Styrofoam.)
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The completed MoNA detector will be calibrated with cosmic-ray events. The neutrons it is designed to detect have an energy range up to 250 MeV and beyond. In order to test the cosmic-ray calibration techniques and the response of MoNA to neutrons in the desired energy range a miniature version of MoNA (micro-MoNA) consisting of 8 detector modules was assembled. The electronics and data acquisition design is similar to the version that will be used in the full MoNA detector.
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| Melanie starts the data acquisition software to take cosmic ray measurements on micro-MoNA. | The photograph above shows a portion of micro-MoNA and it's electronics set up in the high bay area for cosmic tests. |
A part of the first data run taken from micro-MoNA (by Mustafa Rajabali) is shown above. The pulse height (energy deposition) spectra for phototubes on two of the modules is show. The peak is the 20.7 MeV cosmic ray deposition in each bar. Basic testing of the cosmic ray response of micro-MoNA was successful indicating that the full MoNA detector can be calibrated using the natural cosmic ray flux.
Micro-MoNA was moved into the N3 vault for tests using the a 150 MeV/A 36Ar beam. A 1.0 cm thick aluminum target was used to generate a high neutron flux with neutron energies up to 150 MeV. The N3 beam line and target chamber was reconfigured for the tests.
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Mustafa and Melanie begin cabling the detectors for
the test.
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The N3 beam line is reconfigured and the target pot is moved into its new position. |
Online results for the neutron test look good and off-line analysis of the data is under way.
Some results were presented at the APS/DNP meeting in October 2002. Each of the students presented a poster on the work they had done. The posters are given below in PDF format (small) and Powerpoint (large).
First Radioactive Beam Experiment with the Modular Neutron Array MoNA Mustafa Rajabali, Melanie Evanger, Ramsey Turner, Bryan Luther (Concordia College, Moorhead, MN), Thomas Baumann, Yao Lu, Michael Thoennessen, Erik Tryggestad (Michigan State University/National Superconducting Cyclotron Laboratory, East Lansing, MI.) : PDF, PowerPoint
Neutron Testing of the Micro-Modular Neutron Array Melanie Evanger, Mustafa Rajabali, Ramsey Turner, Bryan Luther (Concordia College, Moorhead, MN), Thomas Baumann, Yao Lu, Michael Thoennessen, Erik Tryggestad (Michigan State University/National Superconducting Cyclotron Laboratory, East Lansing, MI) : PDF, PowerPoint
Cosmic Ray Testing of the Micro-Modular Neutron Array Ramsey Turner, Melanie Evanger, Mustafa Rajabali, Bryan Luther (Concordia College, Moorhead, MN), Thomas Baumann, Yao Lu, Michael Thoennessen, Erik Tryggestad (Michigan State University/National Superconducting Cyclotron Laboratory, East Lansing, MI) : PDF, PowerPoint
The main MoNA home page contains detailed information about the MoNA project and collaboration. You may also want to check out the National Superconducting Cyclotron Lab.