Low energy - High energy neutron spectrum mismatch with USRTRACK

Hello Fluka users,

I am trying to measure the neutron fluence energy spectrum of
annihilation neutrons produced by antiprotons incident on a copper
target. I am using USRTRACK to score the neutrons, but when I calculate
and plot the "low" and "high" energy parts of the spectrum together,
there is a large mismatch in the spectrum at 20 MeV (see attached plot).

I am using the 260 group low energy neutron transport library and am
scoring up to 0.2 GeV. To obtain the fluence energy spectrum, I am
multiplying each bin by its energy width in GeV.

I am not sure if my 0.0564 cm thick target qualifies as "very thin" and
would lead to the artifacts mentioned in section 10.1.1 of the Fluka manual.

If anyone has any idea why I am seeing such a disjointed spectrum, I
would greatly appreciate hearing back. I suspect I am making an obvious
mistake somewhere.

Thanks,
Roy

 ***** Antiproton beam on Copper target *****

           DATE: 2/22/ 9, TIME: 1:35:48

          Total number of particles followed 8000000, for a total weight of 8.0000E+06

1

   Track n. 1 "Detn " , generalized particle n. 8, region n. 4
      detector volume: 4.1179E+06 cm**3
      low energy neutrons scored from group 1 to group 260
      linear energy binning from 2.0000E-02 to 2.0000E-01 GeV, 45 bins ( 4.0000E-03 GeV wide)
      Data follow in a vector A(ie), format (1(5x,1p,10(1x,e11.4)))

       9.3207E-05 7.1346E-05 5.8612E-05 5.1491E-05 4.7089E-05 4.3873E-05 4.1383E-05 4.0442E-05 4.0138E-05 3.8976E-05
       3.6574E-05 3.4560E-05 3.2656E-05 3.0891E-05 2.9412E-05 2.7949E-05 2.6460E-05 2.5130E-05 2.3870E-05 2.2665E-05
       2.1429E-05 2.0318E-05 1.9208E-05 1.8188E-05 1.7178E-05 1.6094E-05 1.5152E-05 1.4240E-05 1.3433E-05 1.2556E-05
       1.1785E-05 1.1025E-05 1.0344E-05 9.6611E-06 9.0760E-06 8.4967E-06 7.9397E-06 7.4380E-06 6.9556E-06 6.5730E-06
       6.1461E-06 5.8176E-06 5.4410E-06 5.1268E-06 4.8252E-06

      Low energy neutron data from group 1 to group 260 follow in a vector A(ig), format (1(5x,1p,10(1x,e11.4)))

       1.1050E-04 1.1530E-04 1.1978E-04 1.2528E-04 1.3040E-04 1.3600E-04 1.4196E-04 1.4798E-04 1.5451E-04 1.6047E-04
       1.6755E-04 1.7406E-04 1.8095E-04 1.8897E-04 1.9555E-04 2.0363E-04 2.1193E-04 2.1893E-04 2.2882E-04 2.3712E-04
       2.4614E-04 2.5479E-04 2.6589E-04 2.7594E-04 2.8653E-04 2.9586E-04 3.0583E-04 3.1824E-04 3.3018E-04 3.4113E-04
       3.5399E-04 3.6583E-04 3.7816E-04 3.9259E-04 4.0515E-04 4.1787E-04 4.3224E-04 4.4602E-04 4.5977E-04 4.7452E-04
       4.9029E-04 5.0525E-04 5.2094E-04 5.3430E-04 5.4762E-04 5.5420E-04 5.5419E-04 5.6767E-04 5.8283E-04 5.9744E-04
       6.1432E-04 6.3124E-04 6.5464E-04 6.8922E-04 7.2091E-04 7.4614E-04 7.6509E-04 7.7941E-04 7.9763E-04 8.2241E-04
       8.5900E-04 8.8974E-04 9.2476E-04 9.5912E-04 9.9052E-04 1.0149E-03 1.0309E-03 1.0452E-03 1.0618E-03 1.0838E-03
       1.1161E-03 1.1450E-03 1.1682E-03 1.1901E-03 1.1921E-03 1.2177E-03 1.2088E-03 1.2119E-03 1.2237E-03 1.2353E-03
       1.2468E-03 1.2685E-03 1.2785E-03 1.2907E-03 1.2968E-03 1.3105E-03 1.3185E-03 1.3369E-03 1.3447E-03 1.3478E-03
       1.3578E-03 1.3627E-03 1.3678E-03 1.3756E-03 1.3799E-03 1.3772E-03 1.3811E-03 1.3872E-03 1.3766E-03 1.3719E-03
       1.3674E-03 1.3600E-03 1.3675E-03 1.3638E-03 1.3508E-03 1.3365E-03 1.3308E-03 1.3146E-03 1.3059E-03 1.2855E-03
       1.2763E-03 1.2582E-03 1.2416E-03 1.2170E-03 1.2046E-03 1.1889E-03 1.1815E-03 1.1787E-03 1.1554E-03 1.1423E-03
       1.1316E-03 1.1134E-03 1.0882E-03 1.0693E-03 1.0550E-03 1.0308E-03 1.0123E-03 9.9614E-04 9.6650E-04 9.7209E-04
       9.6004E-04 9.4655E-04 9.3505E-04 9.4260E-04 9.1132E-04 8.9638E-04 8.8621E-04 8.5495E-04 8.5204E-04 8.3648E-04
       8.1819E-04 8.0748E-04 7.9337E-04 7.7539E-04 7.7592E-04 7.5383E-04 7.6143E-04 7.4210E-04 7.2651E-04 7.0712E-04
       7.0778E-04 6.8594E-04 6.9750E-04 6.7100E-04 6.4145E-04 6.1466E-04 5.9510E-04 5.6702E-04 5.4356E-04 5.3070E-04
       5.2734E-04 5.1382E-04 4.9643E-04 4.6803E-04 4.6208E-04 4.5615E-04 4.4248E-04 4.2663E-04 4.2458E-04 3.9501E-04
       3.8908E-04 4.0821E-04 3.8497E-04 3.7363E-04 3.9511E-04 3.6256E-04 3.4567E-04 3.4195E-04 3.2029E-04 3.2361E-04
       2.8815E-04 2.4426E-04 2.5950E-04 2.5296E-04 2.1790E-04 2.2135E-04 1.9436E-04 1.9122E-04 2.0067E-04 1.9363E-04
       1.7672E-04 1.8928E-04 1.7020E-04 1.9424E-04 1.6352E-04 1.3144E-04 1.4770E-04 1.5531E-04 8.9368E-05 1.7402E-04
       1.2475E-04 1.1208E-04 1.1391E-04 1.1004E-04 1.1694E-04 9.7854E-05 1.0994E-04 1.1000E-04 7.7684E-05 5.7431E-05
       5.0455E-05 5.9803E-05 5.7591E-05 3.2866E-05 4.2201E-05 2.7094E-05 1.7394E-05 4.4670E-05 0.0000E+00 3.6824E-05
       0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 1.6503E-04 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00
       0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00
       0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00
       0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00

• chemical/x-gamess-input attachment: pbar-cu-neutrons.inp