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Neutron Testing of the Micro-Modular Neutron Array M. R. Evanger M. R. Evanger aa, M. Rajabali M. Rajabali aa, R. E. Turner , R. E. Turner aa, B. Luther , B. Luther aa, T. Baumann , T. Baumann cc, Y. Lu , Y. Lu bb, M. Thoennessen , M. Thoennessen b,cb,c, E. Tryggestad , E. Tryggestad cc

aa Concordia College, Moorhead, MN Concordia College, Moorhead, MN bb Michigan State University , East Lansing, MI Michigan State University , East Lansing, MI

c c National Superconducting Cyclotron Laboratory, East Lansing, MINational Superconducting Cyclotron Laboratory, East Lansing, MI

ABSTRACT

Eight of the 144 detector modules composing the Modular Neutron Array (MoNA), a large-area neutron detector, were placed in a beam of intermediate energy neutrons produced from a 155 MeV/u 36Ar beam striking a 1 cm Al target at the National Superconducting Cyclotron Laboratory. Micro-MoNA (MoNA), the eight-module set-up, was arranged in two vertically stacked horizontal planes of four modules placed 5 m from the Al target. Each module consisted of a 200 x 10 x 10 cm3 bar of BC-408 plastic scintillator with photomultiplier tubes mounted on each end. Two 1 cm plastic scintillator veto bars were placed in front of each horizontal plane. Tests were conducted with and without a shadow bar. MoNA is designed to measure the energy and position of neutrons. The energy is deduced from neutron's time-of-flight relative to a start detector. The position is calculated from the time difference between the two photomultiplier tubes (PMTs) at each end. This first neutron energy spectrum measured with MoNA as well as the position resolution of the detectors are presented here.

This work supported in part by grants from the National Science Foundation.

Centimeters/channel: 1.87

Calculated Shadow: 11.8 channels = 21.69 cm

SETUPELECTRONICS

45 cm206.6 cm

475 cm

35.5 cm

14.5 cm

START DETECTOR

ALUMINUM TARGET

BRASS SHADOW BAR

BRASS SHADOW BAR DIMENSIONS

7.65 cm

27.40 cm

5.10 cm

EIGHT MoNA BARS

BEAM DIRECTION

VETO BARS

MoNA SETUP

Horizontal Position of Bar 7, No Shadow Bar (no veto)

Horizontal Position of Bar 7, With Shadow Bar (no veto)

CHARACTERISTIC PMT ENERGY DEPOSITION SPECTRA[No veto is applied. Spectra are shown for corresponding PMTs on a MoNA bar. PMT 11 and PMT 15 are on bar 7 (front bar), PMT 10 and PMT 14 are on bar 6 (directly behind bar 7).]

PMT 11

PMT 15

NEUTRON TIME-OF-FLIGHT SPECTRAThe neutron energy is deduced from the measured time-of-flight relative to the start detector.

HORIZONTAL POSITION RESOLUTION

TIME-OF-FLIGHT FOR BAR 7 (without veto)

RENDERING OF COMPLETE MoNA

MoNA

The 155 MeV/u 36Ar beam impinges the Aluminum target and produces neutrons and charged particles. When the brass bar is placed in the production cone a shadow is formed on the MoNA bars, where no particles are detected. Using the shadow bar’s measurements, the shadow dimensions can be calculated and compared with actual data, and the position resolution of the MoNA bars can be determined.

© T. Baumann 2001

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SUMMARY

PMT 10

PMT 14

MoNA will be a large-area neutron detector located at the NSCL. MoNA will have a frontal area of 160 x 200 cm2.

MoNA tests were conducted to determine if the MoNA bars detected neutrons properly and to find horizontal position resolution, energy deposition, and neutron energy.

A more detailed description of MoNA can be found on poster 5P1.071, P.J. Van Wylen et. al., in this session.

The eight MoNA bars used for the MoNA setup were placed behind two 112 x 10 x 1 cm3

veto bars, that did not cover the entire length of the 200 cm bars.

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Measured, with shadow bar

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Ideal, with shadow bar and no background noise

• MoNA showed that MoNA bars successfully detect neutrons.

• The horizontal position resolution of the MoNA bars was found to be no greater than 7.5 cm FWHM.

• MoNA showed that energy deposition and neutron energy can be effectively measured with the MoNA bars.

Thirty-two MoNA bars were assembled and tested at the NSCL.

4 channel difference = 7.5 cm FWHM