Re: Inner plate on bcal

[Elton Smith <elton@jlab.org>]

Hi Zisis,

Thanks for your response.

Tim: I guess we can go forward with refining a design with an 8 mm Al
sheet at the inside of the Bcal.

Cheers, Elton.

On Wed, 6 Feb 2008, Zisis Papandreou wrote:

> Hi Elton:
>
> a few remarks embedded in your text.
>
> On Feb 5, 2008, at 2:34 PM, Elton Smith wrote:
>
> >
> > Hi George and Zisis,
> >
> > We had our weekly meeting with the JLab engineering/design team this
> > morning and discussed bcal installation (along with other issues).
> > There
> > are many good reasons to have an inner skin to the bcal which can
> > be used
> > for mechanical assembly. We have discussed this previously, but
> > wanted to
> > try to pursue this a little further. We have a slightly different
> > proposal
> > now and some questions.
> >
> > The current thinking would be to attach an 8 mm Al skin (equivalent
> > to the
> > first 0.1 rad length of Pb sheet). This has structural advantages
> > over the
> > lead, and also would be symmetric to the Al plate on the outer
> > layer, so
> > you could use the same procedure for attachment.
>
> Fine, sounds ok.
>
> > Here we have some questions or be reminded of how the first layers are
> > constructed: Is the layer of Pb next to the Al base the same
> > thickness as
> > the other layers? How is that it has a flat surface instead of a
> > grooved
> > surface? (We assume that the swaging machine groves the top and
> > bottom of
> > each layer at the same time). Finally, can one put grooves into the
> > Al and
> > use it instead of a Pb sheet?
>
> The very first Pb sheet is glued to the aluminum base plate with a
> two-compound industrial epoxy.  We chose this as something expected
> to be stronger than the BC-600 optical epoxy, and since there are no
> fibers (actually there is only one, see below) between the Pb and Al;
> the extra surface (and porosity?) of the fibers help the BC-600
> adhere better and keep everything together.  This 1st Pb sheet is
> identical in construction to all Pb sheets: it is grooved on both
> sides of course, and has the same 0.5mm thickness.  The Al base plate
> has a single 0.5mm groove (trench) machined along its entire length.
> In it we glue a single fiber using BC-600.  This acts as a "guide
> rail" for the 1st Pb sheet.  Once the BC-600 is cured, we then apply
> the industrial epoxy and glue the 1st Pb sheet; the single fiber
> keeps it from twisting/migrating under pressure when the industrial
> epoxy is setting/curing.  We haven't seen the need to increase from 1
> guide fiber to 3 or 5; 1 seems to do the job.
>
> Grooving the 8mm Al sheet that you are proposing to use at the other
> end (inner side) of the modules, may not be the best way of tackling
> this.  Reversing the above process would have us installing a single
> fiber (or wire of equivalent diameter) in the middle of the topmost
> (last) Pb sheet using BC-600, let it cure, and then use industrial
> epoxy to glue the 8mm Al plate, once it has been machined with a
> single groove for the guide fiber.  We should be able to do this easily.
>
> We have measured module thickness difference of ~1mm from one end of
> the module to the other (over its 25cm thickness).  This is quite
> good, but Tim you have to take into account that the 8mm Al plate
> will not be perfectly parallel to the beam axis, and its inclination
> will be slightly different from module to module (along all three
> axes probably), since no two modules are precisely identical, due to
> the nature of the construction process.   How is this best handled?
> Using varying depths of bolts into threads (we assume you are
> thinking of putting threads into the 8mm Al)?  What about spring-
> loaded mechanisms?  Of course all this needs to be accurate and
> reproducible for the alignment of the tracking packages.
>
> > These questions arise as we would like to see if an inner skin can be
> > added because it increases the mechanical robustness of the
> > calorimeter.
> >
> > Thanks for your patience, Elton.
>
> So the answer is yes on adding an inner skin.  We just have to think
> things through.  On the BCAL side it is a more straight forward
> issue, and we can certainly experiment with some small samples, to
> ensure that the reversed process (fiber on lead first, then plate)
> works as well as the regular process (fiber on Al first, then Pb
> sheet).  The rest is up to you Tim!  :-)
>
> Cheers, Zisis...
>
>
> >
> > ---------- Forwarded message ----------
> > Date: Tue, 5 Feb 2008 14:31:49 -0500 (EST)
> > From: Elton Smith <elton@jlab.org>
> > To: Tim Whitlatch <whitey@jlab.org>
> > Cc: Elton Smith <elton@jlab.org>, Elke Aschenauer <elke@jlab.org>
> > Subject: Inner plate on bcal
> >
> >
> > Hi Tim,
> >
> > I checked the radiation lengths of materials to see how much we can
> > afford
> > to use for the inner plate of the calorimeter:
> >
> > rad len Al = 8.9 cm
> > rad len Fe = 1.76 cm
> > rad len Pb = 0.56 cm
> > rad len W  = 0.35 cm
> >
> > We are nominally starting out with 0.5 mm Pb (0.09 rad len). So
> > adding/replacing with another material of roughly the same
> > thickness in
> > rad lengths is reasonable. This corresponds to 8 mm of Al and 1.6
> > mm of
> > Fe (i.e. stainless). This is somewhat thinner than what we were
> > considering this morning, but I wanted to get your feedback on how
> > useful
> > this would be.
> >
> > If Al is chosen, it was additional advantages. First it will be
> > relatively
> > thicker, so attachments are easier. Also, it may be possible to
> > groove the
> > Al sheet, which might allow us to replace the first layer of Pb and
> > therefore increase the amount of backing structure. Finally,
> > machining Al
> > along with the rest of the pyramid should be very straightforward.
> >
> > Let me know what your thoughts are regarding Al/Fe before I run
> > this by
> > our Regina friends.
> >
> > Thanks, Elton.
> >
> >
> > Elton Smith
> > Jefferson Lab MS 12H5
> > 12000 Jefferson Ave
> > Suite # 16
> > Newport News, VA 23606
> > elton@jlab.org
> > (757) 269-7625
> > (757) 269-6331 fax
> >
>
> ---
> Dr. Zisis Papandreou		|  email: zisis@uregina.ca
> Department of Physics	|  tel. : (306) 585-5379
> University of Regina		|  tel. : (306) 585-4149
> Regina, SK  S4S 0A2		|  fax. : (306) 585-5659
> World Wide Web:    http://www.phys.uregina.ca/sparro/zisis/
>
>
>