Large Acceptance Neutron Detector

This page describes the large acceptance neutron detector in place at the University of Notre Dame for use with the FN tandem accelerator beamline and the TwinSol apparatus.


neutron wall



neutron wall schematic



  • The 26Mg(3He,n) run executed in December 2008 showed that at the neutron production rates accessible the background due to cosmic events produces a considerable error in the measured number of neutrons throughout the deposited energy spectrum. This fact is easily see below in the Update dated 5/15/09. The neutron peaks appear to the left (longer times) of the prominent gamma flash peaks. They are seen to be slightly above background. Since the cosmic rays are typically minimum-ionizing particles (muons), one can use the fact that these are likeley to deposit a constant amount of energy per distance in plastic scintillator in order to reject such events. To do this, a scintillator sheild to the wall is to be constructed and used as a veto on the wall events. If the sheild scintillator is rather thin it is unlikely that a signal neutron will deposit energy in both the neutron detection bar and the shield, but it is highly likely for cosmic rays. A link to a page concerning the development of this detector follows.


    • Cosmic Ray Veto Counter [HTML]


  • The neutron wall has been used in two runs in preparation for a 76Ge run which will help assess the pairing strength in nuclear matrix elements which are important for neutrioless double beta decay. These runs were done in May of 2008 and December of 2008. The May run was at a distance (relevant for time of flight) of approximately 6 m from the target and using the reaction d(d,n)3He for calibration and efficiency purposes. The second run in December utilized the 26Mg(3He,n)28Si reaction at a distance of about 15 m in order to gain an understanding of the background that will be present in the 76Ge experiment. A survey of the angles and distances to the scintillator bars in each situation is seen below.


    • 6 Meter Position:


    Table: Measured angles for neturon wall scintillator bars.
    Bar Label $ \angle$ (deg) $ \angle$ error (deg) $ \angle$ (deg) $ \angle$ error (deg) $ \angle$ (deg) $ \angle$ error (deg)
    2 7.56745 0.0183896 - - 6.5 1
    3 9.24723 0.0183697 - - 8.5 1
    4 10.9332 0.0183947 - - 10 1
    A 13.6351 0.0190689 - - 14 1
    B 15.5743 0.0194401 - - 15 1
    C 17.4736 0.0199961 - - 16.5 1
    D 19.3007 0.0205429 - - 18 1
    E 21.0277 0.0209452 21.0631 0.0280229 20 1
    F - - 23.5134 0.0274787 22 1
    G - - 25.2335 0.0274135 24 1
    H - - 27.2796 0.0274186 25.5 1
    I - - 29.1566 0.0274463 27 1
    5 - - 33.9076 0.0277379 34 1
    6 - - 35.4105 0.0279289 36 1
    7 - - 37.33 0.0282839 37 1
    8 - - 39.126 0.0286227 39.5 1




    15 Meter Position:


    Table: Measured angles for neturon wall scintillator bars.
    Bar Label $ \angle$ (deg) $ \angle$ error (deg) $ \angle$ (deg) $ \angle$ error (deg) distance (m) distance error (m)
    2 7.53268 0.0260612 7.53324 0.0283783 14.629 0.003
    3 8.21049 0.0282746 8.22816 0.0283702 14.638 0.003
    4 8.99218 0.0272066 8.97731 0.028343 14.682 0.003
    A 10.343 0.0272074 10.327 0.0283312 14.705 0.003
    B 11.0227 0.0272111 11.0206 0.0283284 14.701 0.003
    C 11.7932 0.0272167 11.7923 0.0283257 14.697 0.003
    D 12.5045 0.0272264 12.5021 0.0283238 14.7 0.003
    E 13.2583 0.0272295 13.2583 0.0272295 14.703 0.003
    F 14.7521 0.0272468 14.7506 0.0283266 14.703 0.003
    G 15.4071 0.0272549 15.4717 0.0283262 14.705 0.003
    H 16.1435 0.0272716 16.2047 0.0283285 14.706 0.003
    I 16.8954 0.0272822 16.9324 0.0283312 14.714 0.003
    5 19.0985 0.027325 19.0957 0.0283449 14.747 0.003
    6 19.7829 0.0273491 19.8706 0.0283641 14.746 0.003
    7 20.5155 0.0273594 20.5031 0.0283635 14.74 0.003
    8 21.2332 0.0273339 21.2389 0.0283758 14.743 0.003





    Updates




  • (05/15/09) The essential characteristics of the December 2008 run using the 26Mg(3He,n)28Si are embodied in the timing spectra. By time-of-flight (ToF) method it is the approximately 1 ns timing resolution which will allow separation of the ground and first excited states in the 76Ge experiment. The peak appearing at approximately 125 ns is a neutron peak while the primary gamma peak (from the gold beam stop) appears at approximately 325 ns. There is also a secondary gamma peak at approximately 375 ns. The timing detector uses a common stop mode and therefore things which appear at a larger time come earlier.



    • neutron wall timing spectrum



    Sample Timing Spectrum with energy deposited greater than 600 channels (out of 4096~25 MeV) [PDF] [PS] [EPS]



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