[Halld-tagger] [EXTERNAL] notes from tagger working group meeting this morning

[Justin Stevens]

Hi Richard, All,

Thanks for the summary of yesterday’s discussion and sorry I had to leave a bit early for the DNP practice talks.  While we continue to investigate the exact reason for the loss of light yield, I suggest we begin planning a test that can be performed in 2026 to evaluate the options for potential replacement of the scintillating fibers before GlueX-III.

One possibility would be a small test setup on one of the PS arms with candidates for the 2x2 mm scintillators that can be tested with real signals from energy-tagged electrons.  We would need an equivalent readout to the existing TAGM bundles, but most of the electronics infrastructure: f250 and f1TDC boards, crates, LV, etc. should already be available in that area for the PS/ST/TPOL readout.  So, I believe we would need the TAGM preamp boards with SiPMs and light guide fibers to couple to the candidate scintillators we want to evaluate.  Do we have spares of those items that could be used for such a test starting in early 2026?

There is also the potentially more important question of what are the options for replacement scintillators with the appropriate shape that may be more radiation resistant and how can we test that radiation hardness?  I understood from Malte that there may be some LYSO crystals with an appropriate profile and Eljen https://eljentechnology.com/products/fibers may have some appropriate fibers, but we should consider procuring these and other options soon if we aim to test them next year.

More thoughts, suggestions, corrections are welcome.  I just think we need to keep brainstorming on this to keep up the momentum towards finding a solution for GlueX-III.

-Justin

On Oct 9, 2025, at 2:20 PM, Richard T. Jones via Halld-tagger <halld-tagger at jlab.org> wrote:

Hello all,

We had a special meeting this morning at 10:00 devoted to a review of everything we know regarding the degrading response of the tagger microscope over the last few run periods. A new logbook entry has been added containing a historical record of the photon yield and other calibration constants for the TAGM which presents clear evidence of significant loss of scintillation photon yields over the past 5 years, with most of the loss occurring in the rows that saw the greatest flux of electrons.

  *   gain and photon yield history of the tagger microscope<https://logbooks.jlab.org/entry/4459304>

Ideas to further investigate the exact reasons for the decrease, as well as ideas for recovering and sustaining good performance during Phase 3 running are still in process, with follow-up to happen in the next Photon Beam working group mtg next week.

-Richard Jones
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