Date: Thu, 03 Oct 2013 09:55:16 +0900
From: John Flanagan <john.flanagan@kek.jp>


Minutes:

Spectra (John)
--Want spectrometer to be able to discriminate peaks in range of
10-35 keV

Updates:
--Layout (Julie)
--Media converter (Gary)

Brainstorming topic:  Energy measurement using Compton-scattering off
LER/HER be
ams.
--Need a detector that can measure energy of single photons in
few-to-tens of Me
V range.
--Accuracy needed for absolute beam energy measurement
--Precision needed for beam tuning and energy spread measurement
--Liquid He-cooled Ge provides 1e-3 accuracy
--Other possibilities?
----Gary points out that electron yields for 10 MeV gamma in
semiconductors is o
nly in 1e3-1e4 range, so better precision is unlikely from
semiconductor-based a
pproach.


I am also appending mail from Uli on the subject, to get everyone up
to speed on what he is thinking.
(Another one will follow in the next mail.)

Cheers,
John

-------- Original Message --------
Subject: Energy measurment
Date: Fri, 30 Aug 2013 14:08:37 -0700
From: Wienands, Uli <uli@slac.stanford.edu>
To: john.flanagan@kek.jp <john.flanagan@kek.jp>
CC: Wienands, Uli <uli@slac.stanford.edu>

Hi John,

I have now written down my thoughts on the energy calibration using
inverse Compton scattering. Please find attached a draft (pdf).

The salient points are:

We need to inject the photons at a relatively small angle against the
beam direction (i.e. in the lab the photons come sort-of from
behind).

That angle causes a sensitivity in energy that I am getting rid of
by, in essence, measuring this angle using the beams. Possibly the
most controversial aspect of this proposal.

The energy accuracy I estimate, 180 and 260 keV, should be realistic
assuming my angle-measurement procedure works. But it'll be hard to
do much better; basically, one hits the limit of the detectors.

Nothing in the hardware is particularly exotic.

Anyway, I am soliciting your comments esp. wrt. this being credible
and also this being realizable in the SuperKEKB context (space,
penetrations etc.). Note that I have not put any costs against this
yet. I would guess, without such thought, that it'll end up being
something like a couple M$. We have done similar things at SLAC so I
intend to pick the brains of people who know about lasers and beam
guides. For the vacuum components I would want to ask you for a KEK
estimate.

The physics case in this document is weak/non-extant. Once we think
we have something we can build and want to propose, do you think Gary
Varner would be willing to add some physics justification?

Thanks,

Uli