TB subgroup mtg June 8, 2023

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Agenda

The point has been raised that are baking in the location of the halo monitor target, without understanding what measurement will be made with that target. This is time critical, as the beamline design is mature. After looking over the case, I feel the design is not sufficiently reviewed to be ready to bring before the full TB. I am requesting that this subset of the TB take a look first. To prime this disucussion, I’ll note that one particular point of concern for me is that a vertical target motion may make it difficult to explore a horizontal tail, whereas dispersion of an energy tail would seem to be the most obvious mechanism for generating a beam halo.

I would like to have a meeting to address the following questions:

  • What measurement would we make with a halo monitor, and how (note the current location of the halo monitor target is quite close to the MOLLER target chamber)?
  • How important is this system? If we fail to make a functional system, what do we lose?
  • Is the current specification (last updated in 2017, attached below) sufficient?
  • If we need the system, what do we need to do (for example: what questions to be answered in simulation or data review?) to convince ourselves that we will have a functional tool?


Slides from Mark docdb:1136

The halo target specification document pdf

The song sheet for the MOLLER Hall A beamline, with the currently specified halo target position highlighted. songsheet

Minutes

  • A halo monitor has never been needed in the previous Hall A experiments. However, it proved important in Qweak, and no other experiment has combined small angle and open acceptance spectrometer before.
    • We conclude that past experiences is that open spectrometer parity-violation experiments at high precision require a halo monitor
  • There are two separate systems referred to as "halo" measurements
    • detectors near the beam line designed to measure scattering from upstream, without a specific target. These are similar in concept to the Compton "finger" detectors.
    • A "halo target" with a limited beam aperature through a radiating metal plate, with detectors placed at small angles nearby to measure scattered particles from interaction of halo with the radiator.
  • Halo target system
    • Halo target detectors mounted near the production target are planned.
      • These would be mounted to the beampipe. No shielding is planned, which would require that the production target should be removed.
      • Q: is the full power beam without a production target allowed on the dump?
      • Is there really space for detectors there? Can we get them into the CAD to hold the space, plan for cable runs?
    • A question was raised about synchrotron light from the arc bend or the compton chicane into the halo target. This was not a noted effect during Qweak, but the beam energy was much lower for that experiment.
      • Note on synchrotron light from the Hall A Compton polarimeter PRST-AB
      • Will synchrotron light confuse the planned measurement of the beam halo?
    • How sensitive will these be? - Devi will simulate
    • What will be the backscattered backgrounds? Can this be used (even with the backscattered backgrounds) with the production target in place? Will it at least be useable without the production target? - Devi will simulate
    • Should the target have horizontal drive to better explore an energy tail?
    • If rectangle aperture, how large in vertical, and in horizontal? How thick?
    • How do we handle the FSD for this? One possible mode of operation might ask for movement of this target with beam. A calibration may require very low current beam to directly intercept the metal. Are these compatible with the necessary FSD?
  • Halo monitor detectors
    • detectors are currently located near the beamline just downstream of the BCM pair.
    • Q: are these better located downstream of the narrowest aperture (the raster pipe)?
    • Q: how much backscattered background will they see
    • Q: do we have readout for these already?
    • Devi will test the presently installed detectors.

Attendance

Mark Pitt, Dave Armstrong, Ciprian Gal, Paul King, Bob Michaels, Devi Adhikari, Caryn Palatchi, Kent Paschke

Followup

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