Microball's New Partner
(This page was created by D.G. Sarantites and was last updated on March 14, 1999.)
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The Si Wall was designed especially for use in conjunction with the Microball and Gammasphere for the purpose of doing discrete-line prompt proton and a-particle spectroscopy. The Si Wall is somewhat different in purpose from the larger array called LASSA, in that it uses the thicker E Si-strip detectors and at present has no backing by CsI(Tl) scintillators.
The LASSA array was constructed by a collaboration between the Indiana University (R. DeSouza), Washington University (Lee Sobotka, J. Elson, D.G. Sarantites, et al.) and Michigan State University (W. Lynch et al.) This acronym stands for Large Area Silicon Strip Array. Once upon a time someone had a Killer Virus called LASSA !!!
A set of 128 charge sensitive preamplifiers have been assembled in a compact geometry. They are arranged in the shape of a cube for compact packing that allows closest distance from the target and just outside the scattering chamber or the Gammasphere Al shell.
A close-up view of one of the preamplifiers is seen here. This
custom pre-amplifier was designed at Indiana University in 1998.
About 400 of these were assembled at I.U.
A close-up view of the preamplifier Cube is seen here.
A medium-resolution
view (800x568) of the preamplifier cube is available.
Substantial amount of heat is produced by 128 preamps cramped in a small volume that cooling by forced air flow is required.
The arrangement of the preamplifier cylinder and the cables tube can be seen (in a rather fuzzy picture) where Lee Sobotka is connecting the preamplifier box outside the Microball chamber in Gammasphere.
Each detector has 16 strips and an area of 5cm by 5cm.
The detector set consists of a variety of DE
and E detectors. The real DE detectors
are 65 microns thick and are all one-sided for the readout. A set
of 9 detectors 500 microns thick are double sided in readout.
Finally, a set of 6 detectors are 1000 microns thick and are one
sided. In the high energy applications the 65 and 500 microns
detectors are used backed with thick CsI(Tl) scintillators. For
the Gammasphere applications only four of the 65 and 1000 microns
telescopes are used.
In the following pictures we will demonstrate some of the assembly characteristics for the Si array.
A detail of the cable connection to the Si wafer is shown to
the right. The Si wafer is located on the right and it produces a
reflection of the cable.
The thin wires making the bonds to the Si strips are seen to the
right.
The connection of the thin ribbon cable to the back of a Si
wafer is shown to the right.
A one-sided wafer connected to its cable is seen from its
front.
A two-sided wafer is seen in front view. It is connected to
both cables.
Examples of isotopic resolution of intermediate mass fragments is included here: DExE map as a PDF file. 1,2,3H, 3,4,6He, 6,7,8,9Li, 7,9,10Be, and 14,15,16N, isotope resolution is clearly seen. The data are from the reaction of 11 MeV/A 60Ni on 100Mo. The hydrogen and He isotopes punch through significantly, while the Li isotopes punch through to a lesser extent.
The Wall consists of four DExE Si
telescopes mounted in a special arrangement that packs closely
around the beam axis.
The Si Wall is seen straight-on. The DE and E detectors are connected to their
cables and are mounted in the support structure that allows
positioning at the beam exit from the Microball. A medium-resolution
(554x490) somewhat larger view is available here. Note the
staggered arrangement and the 2-axes tilt of the detectors under
closest possible packing.
Next we have to put it in front of the Microball and in the
chamber.
Oh Sh.., it does not fit! The cables are not long enough. Now what?
Well, we put it too far downstream and the cables will not reach.
No problem, move the Si Wall back and closer to the
Microball where it should be.
A high resolution
(1536x1024) picture is available here.
Wow, it does fit!
A photograph showing the Si Wall and the Microball in the
Gammasphere chamber. This is a back view showing the entrance
collimator. The twisted pair cables bring the Si signals out.
The cables are exiting the chamber from a backward angle. The
beam enters from the left.
A high resolution
(1536x1024) picture is available here.
Another photograph showing the Si Wall and the
Microball in the Gammasphere chamber. This is a front view
showing the Wall a little better.
Crowded is n't it? The twisted-pair cables bring the Si signals
out. The cables are exiting the chamber from a backward angle.
The beam enters from the right.
A high resolution
(1536x1024) picture is available here.
Now everything is connected, buttoned up and ready to go. A
photograph showing the Si Wall and the Microball all in
the Microball chamber. The Si-Wall tube (left) has a
cylindrical enclosure outside the Gammasphere Al shell that houses the
preamplifiers. They get quite hot, so air is pumped into the cylindrical
housing providing adequate cooling of the preamplifiers. The exit holes
for the cooling air can be seen in the high resolution picture.
A few of the neutron detectors are barely seen on the front of
the Gammasphere. The beam enters from the left.
A high
resolution (1536x1024) picture is available here.
Due to our rather high density specially designed electronics the setup is very compact. This is primarily due to the Shaper-Discriminator modules. They were designed and constructed by Jon Elson at Washington University.
The Washington-University Shaper-Discriminator module is a variant of the Microball signal processing module adapted for Si detectors. It contains all the necessary functions for 16 detectors (here strips).
The modules are constructed on two 6-layer boards to a size suitable for CAMAC crates ferom where they receive power and are read or downloaded with gain and threshold information.
The modules have a base line restorer and an adjustable pole-zero correction for each detector channel. In addition, the following functions for 16 channels are available as outputs in a 34-pin connector:
Eight of the Washington-University Shaper-Discriminator modules
sufficient to process all the signals from the Si Wall.
A high resolution
(1530x1024) picture is available.
Here Bob Charity and Matt Devlin are checking
out the Si-Wall signals. Behind Bob are the Neutron
detector FERAs. Left of the oscilloscope is the Microball rack of
electronics and on its right is the rack with the Si-Wall
modules (1 CAMAC crate) and the FERA ADCs (1 CAMAC crate also).
A high
resolution (1536x1024) picture is available here.
Immediately behind the scope is the Gammasphere electronics
rack (FMA not used here). At the center of the picture are the 2
racks with the Microball and the Si-Wall electronics. The
rack on the right houses the Neutron detector FERAs. The
remaining of the neutron detector electronics are located outside
the shack.
Now everything is running so D.G.S. can take pictures.
A high resolution
(1536x1024) picture is available here.
The Si Wall had its maiden experiment on December 10, 1998. It was used in conjunction with the Microball in an experiment to verify and extend the discrete proton decay from a deformed band in 58Cu to a spherical shell model state in 57Ni. The experiment worked very well and data were obtained with twice the statistics than the earlier experiment that lead to the observation of this effect.
The first experiment performed with the Si Wall and the Microball was:
The setup had 80 Ge detectors in Gammasphere, 20 Neutron detectors and 68 detectors in the Microball (the 3 most forward detector rings were removed). The Si Wall with four DExE 16-strip telescopes covered the 6o - 44o forward angles.
( Back to the Microball Page).
For comments and corrections send mail to: dgs@wuchem.wustl.edu.
