[Halld-tagger] beamline meeting this morning at 11:30

[Richard Jones]

Hello Ken and Peter,

With the loss of the beam this week, I have finally had a moment to go back
and tackle my backlog of unanswered questions regarding the beamline. Back
in early January, you wrote to me with a specific question about how to
translate between our two angle conventions. Now, with the help of your
animated gif, I am able to provide what I think is a definitive answer.
First some definitions.

   - I call the Glasgow secondary angle theta_ortho because it is called
   the orthogonal angle in the animated gif labels. Its counterpart in the
   Glasgow convention I will call theta_edge, which is the Glasgow angle that
   sets the location of the primary (2,-2,0) coherent edge. So theta_ortho,
   theta_edge, and phi are the three angles that set the orientation of the
   diamond in the Glasgow code.
   - The UConn code uses a theta_small for the angle that sets the location
   of the coherent edge, and the complementary tilt angle I call theta_large.
   Of course one may set theta_large < theta_small, they are just labels that
   indicate their intended use in the code. So theta_large, theta_small, and
   phi are the three angles that set the orientation of the diamond in the
   Glasgow code.

After a detailed study, I now conclude that our two definitions are
identical, that is theta_ortho == theta_large and theta_edge == theta_small
in terms of their definitions. To show that this actually works, I have
done a special exercise with my code, and created an animated gif that I
can compare directly with yours. Recall that the UConn code is a generator,
not an analytical formula, so it takes a lot of Monte Carlo to get
sufficient statistics for a detailed comparison. Because of this, I created
a toy integrator that takes the kernel from my generator and creates an
approximate curve to describe the distribution that it would generate in
the large N limit. There are some approximations that I used in this.

   1. Only lattice vectors with h,k,l in the range [-10, 10] are included.
   Your code generates peaks outside this range, perhaps as large as [-20,
   20], but I thought that [-10,10] would be sufficient to test the
   equivalence of our angle definitions.
   2. All of the edges are smeared with the same smearing function as the
   (2,2,0) vector would be. This simplifies the code that generates the peaks,
   but underestimates the broadening of the peaks at higher order. Those
   should be smeared by an additional factor of the order multiplier: eq.
   (4,0,0) edge is under-smeared by factor sqrt(2), (6,6,0) is under-smeared
   by factor sqrt(18), etc. But the edges should all be in the right place,
   just somewhat sharper than they would be in real life.
   3. We are probably using somewhat different values for some physical
   constants, because our edges are not at exactly the same place for the same
   angle settings. But the patterns are the same, and the evolution of the
   peaks is unmistakably identical. I give my exact values for the physical
   constants below, so you can compare with what you have in your code.

Here is a link to my animated gif, so you can have a look for yourself and
see if you agree.
https://urldefense.proofpoint.com/v2/url?u=https-3A__drive.google.com_file_d_1F-2DhT7rM8ikzJVNMDmODWsCXKH391mnZF_view-3Fusp-3Dsharing&d=DwIBaQ&c=lz9TcOasaINaaC3U7FbMev2lsutwpI4--09aP8Lu18s&r=lneuPoR_Y_qxqVHZY6VsYU8W43i3GcK7nXR2epWlm0Y&m=U5TV4QD4zS6w7-iHN4pT2AmN9IqSKGEtBOY0a9P7JHA&s=wBPkKRyDPtY4mC8H426xeRxfU8SMQuicZarjIg5fLko&e= 

My physical constants are: (energy,momentum in GeV, length in m)

   - mElectron = 0.510998910e-3;
   - hbarc = 0.1973269718e-15;
   - diamond lattice_constant = 3.5668e-10;
   - diamond thickness = 5e-5;
   - collimator diameter = 0.005;
   - radiator-collimator distance = 76;
   - endpoint energy = 12;
   - theta_small = 3.5e-3;

>From now on, I propose that we decide on a common naming scheme for these
angles. I am happy to call them theta_ortho and theta_edge, if that works
for you. With this settled, we should be ready to start production of some
simulations. Please send me your order.

-Richard Jones

On Mon, Mar 5, 2018 at 9:30 AM, Ken Livingston <
Kenneth.Livingston at glasgow.ac.uk> wrote:

> Hi Richard,
>
> I'm on strike today (literally, not metaphorically), so won't attend. Did
> you manage to have a look at the reason for the discrepancy between your
> crystal angles and mine yet?
> Regards,
> Ken
>
>
>
> On 05/03/18 10:29, Richard Jones wrote:
>
> Dear colleagues,
>
> Please remember our photon beam meeting this morning starting at 11:30. I
> have created an agenda page at the usual place on the wiki. I hope that I
> will be able to arrive at JLab in time for the start of the meeting. If
> not, please start without me, I will come as soon as I get in.
>
> https://halldweb.jlab.org/wiki/index.php/BLTWG_Meeting_03/05/2018
>
> -Richard Jones
>
>
> _______________________________________________
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