PREX/CREX Thursday, August 8, 2013 10:00am EST

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Last meeting: PREX/CREX Thursday, August 1, 2013 10:00am EST


  1. Updates from Rakitha haplog 2895, haplog 2896
  2. Updates about meetings with engineers


Attending: Juliette, Rakitha, KK, Kent, Paul, Lorenzo, Seamus

  1. Paul has noted that compilations of GDR from photonuclear studies show large resonances up to 20 or 30MeV. These are especially significant for copper, but relatively small for tungsten. If this implies ejection of neutrons at the GDR cross-sections with energies comparable to the incident photon, they would be very problematic for our shielding strategy, which was based on not having to worry about significant neutron flux above 10 MeV.
    1. more notes below
    2. This strongly suggests that the 70/30 W/Cu alloy may be problematic.
    3. A more general concern is whether GEANT4 properly simulates the 10-40MeV neutron production: are the GDR cross-sections and models for neutron ejection physically accurate? Methods to benchmark were discussed. If we could get data to compare to G4 output, that would be helpful. Or if we could get a table or plot of the cross-sections used by G4.
    4. Or... should we design a beamtest. Running an e-beam into a block of W and/or Cu, with a neutron detector shielded by poly, might be useful, but requries careful design. And, also, we have to figure out how to detect neutrons.
  2. Examined haplog:2895, 2896.
    1. There appears to be an error in 2896. Power deposition goes up as length increases. Looking at figures, it appears that the front aperture changes as length increases, so a smaller effective angle bite.
    2. Normalizing to deposited numbers, it appears that there is little benefit beyond 25 R.L. length length and 12 R.L. radius.
    3. Slit scattering (beamline but originating from collimator) will be more significant than punch through. It should be addressed later using downstream collimator or increased conical output.
    4. Use of W vs W/Cu should also improve slit scattering
    5. Our next task is to define the alignment sensitivity. We should move the collimator slightly to the side, to evaluate both the change in power on the collimator and the change in power dumped in the hall. To do this, I think we will need to use a detector downstream of the collimator (a "telescope" to the dump) or, alternatively, define a critical aperture somewhere.
  3. After the meeting: Paul mades some phone calls. His notes are attached:
    1. I talked with Henry Weller and Albert Young about neutrons from GRD.
  • The maximum neutron energy is the photon energy minus the neutron binding energy,

which comes from the masses of the isotopes.

  • For heavy nuclei, the GDR heats up the nucleus, and low energy neutrons evaporate.

Even for higher energy photons, most of the neutrons would be low energy.

  • Some neutrons are knockouts, which would have high energy. I would expect them to be rare.

In other cases, things like alphas are also emitted. The accompanying neutrons must have low energy.

  • The neutron yield can be predicted to a few % or better with the code MCNPX.

A graduate student can learn to use the code in a few months. It is $3K for professionals.

  • There are some useful rules of thumb. The GDR energy goes like 80/A^1/3.

The total integrated cross section goes like 60 NZ/A mb-MeV.

  • How accurate GEANT is is not known. Given the new information, I am less concerned. Quite a few

high neutrons are produced.

  • [kdp: These last two sentences should be interpreted as: Paul is less concerned, because the G4 output appears to show a reasonable rate of higher-energy neutrons, based on what he has learned about photonuclear neutron production.]

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