Large Area Silicon Strip Array

The LASSA array was constructed by a collaboration between the Indiana University (R.T deSouza, B. Davin), 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. LASSA is also the name of a killer virus (hemoragic fever) -- how appropriate!

1. Overview
2. The Preamplifiers
3. The Detectors
4. Examples of Energy Spectra and Isotopic Resolution 
5. The Si-Wall in Gammasphere
6. Signal Processing and Acquisition
7. Experimental Milestones
8. Nuclear Dynamics Group homepage

1. Overview

LASSA consists of 9 individual telescopes which may be arranged in a variety of geometries. This array was built to provide isotopic identification of fragments (Z < 10) produced in low and intermediate energy heavy-ion reactions. In addition to good isotopic resolution it was essential to provide a low threshold for particle identification as many of the fragments emitted in these reactions are low in energy. No comparable detector array existed (or exists) when we started the LASSA project. LASSA was initially funded by the U.S. Dept. of Energy (Indiana University, Washington University) with subsequent funding from the National Science Foundation (Michigan State University).

Each LASSA telescope is composed of a stack of two silicon strip detectors followed by 4 CsI(Tl) crystals. As seen in the figure of a telescope at the right, the silicon which faces the target is 65 microns thick while the second silicon is 500 microns thick. Both silicons are ion-implanted passivated detectors, Si(IP), purchased from Micron Semiconductor. While the silicon design is simply the Design W available from Micron, the packaging (frame, cables, etc) was developed for LASSA at Indiana. The 65 micron Silicon wafer is segmented into 16 strips which are read out individually. The 500 micron wafer is segmented into 16 strips on its junction (front) side while the ohmic surface (rear) is segmented into 16 strips in the orthogonal direction. Collection of holes and electrons in orthogonal directions provides two dimensional position sensitivity from this detector alone. The additional position information from the 65 micron detector is used as a redundancy check. The pitch of the detector is nominally 3 mm with a 100 micron inter-strip gap. Behind the silicon detectors are 4 independent 6-cm CsI(Tl) crystals to stop penetrating particles. Scintillation caused by ionizing particles impinging on these scintillators is detected by 2cm x 2cm photodiodes (PD). The signals from the PD are amplified by pre-amplifiers housed in the detector housing.

Back to the Top.

2. The Pre-amplifers

LASSA requires a total of 468 charge sensitive preamplifiers (9x48 for Si and 9x4 for CsI(Tl)). Dense packing of these pre-amplifiers required that they be constructed almost exclusively from surface mount components. Each preamplifier resembles a miniature PC card with 9 pins. Multiple preamp cards are assembled on a "motherboard". Each "plane of strips" for a telescope has its own motherboard. The three motherboards processing signals from a single telescope are mounted in an aluminum cube. This cube is actively cooled to dissipate the approximately 300 mW of heat generated by each pre-amp. 

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 fabricated by the Electronics Instrumentation Services shop in the Department of Chemistry  I.U.

A close-up view of a stuffed preamplifier motherboard is seen here.

Back to the Top.

3. The detectors

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.

The assembly

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 50 micron aluminum  wires making the bonds to the Si strips are seen to the right.
The connection of the thin ribbon cable (all fabricated at Indiana University by B. Davin) to the back of a Si wafer is shown to the right.
A one-sided wafer connected to its cable as seen from its front.
A two-sided wafer is seen in front view. It is connected to both cables.

Back to the Top.

4. Examples of Energy Spectra and Isotopic Resolution

Following the calibration of our Cd + Mo experiment (Oct. 98) the excellent resolution of LASSA is evident.

DExE spectrum of the two Si detectors following energy calibration but without correcting for the thickness variation of the 65 micron detector.
Two dimensional thickness correction map for the 65 micron detector. Notice the large thickness variations particularly at the detector edges.
DExE spectrum of the two Si detectors following energy calibration and thickness correction for the 65 micron detector. Isotopes 6,7,8,9Li are clearly visible as are 11,12,13,14,15C in this representation. Isotopes for all elements upto F are identifible.
DExE spectrum of the Si-CsI(Tl) detectors following energy calibration and thickness correction for the 500 micron detector. Isotopes 6,7,8,9,11Li are clearly visible as are 7,9,10,11,12Be, 11,12,13,14,15C in this representation. Notable is 8B line visible in the spectrum. The Isotopes for all elements upto F are identifible.
An example of some PID spectra illustrating the isotopic resolution for C and O isotopes in a typical LASSA telescope.

Back to the Top.

5. The Si Wall with The Microball in Gammasphere

LASSA can also be configured as a Si Wall to be used in nuclear spectroscopy experiments with Gammasphere. It was originally configured in this mode by D. G. Sarantites (Washington University) in November, 1999. In the first experiment conducted at ANL with the "Si Wall", it consisted of four DExE Si telescopes are 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.

A photograph showing the Si Wall and the 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.

Back to the Top.

6. Signal Processing and Acquisition

The LASSA detectors required the development of a high density electronic system capable of processing the signals. A schematic of our electronic setup for processing one plane of strips is shown at the right.

Central to our high density electronics is the Shaper-Discriminator module. This module was designed and constructed by Jon Elson at Washington University.

The Washington-University Shaper-Discriminator Modules

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 from 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:

1. Preamplifier Inputs via a ribbon 34 pin connector.
2. Leading edge discriminators, with computer controlled or settable thresholds.
3. Linear outputs through a dual shaper. The amplitude of each channel is adjustable via computer controlled attenuators.
4. Attenuated linear outputs from the shaper, with computer controlled gain. The attenuated signal is at preselected fixed ratio to the other linear output. This feature allows for low-gain and high-gain processing via separate ADCs.
5. Time-to-FERA converter outputs for digitizing the individual discriminator times.
6. ECL discriminator outputs for each channel.

1. The 16-channel discriminator logical OR.
2. A 16-detector multiplicity output.
3. A discriminator logical Test output for the computer selected channel.
4. A linear test output for the computer selected channel.

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.

Logic and Digitization

Back to the Top.

7. Milestones

The first experiments

LASSA  had its maiden experiment in June, 1998 at Texas A&M University's K500 cyclotron. It (the first four telescopes) was used to study emission of particle unstable fragments (including neutron unstable fragments) from residues in Ni + Mo at 11 MeV/u (Charity, Sobotka). First results of this experiment have been presented and are now close to publication. LASSA (now consisting of all 9 modules and associated electronics) was used in a campaign of 4 experiments at Michigan State-NSCL's K1200 cyclotron. These experiments examined the influence of temperature, shape, and isospin on multifragmentation.  

• "Decay Characteristics of Thermally Induced Multifragmentation of ¹²C, ³⁶Ar + Au at E/A = 150 MeV" (Indiana)
• "Probing the 'Freeze-out' Mechanism for Multifragmentation Processes of ¹²⁹Xe + Au at E/A = 50 MeV" (MSU)
• "Isospin Equilibration and Fragment Emission Order in the Reactions ^106,114Cd + ^98,92Mo at E/A = 50 MeV" (Indiana)
• "Temperature, Impact Parameter and Isospin Dependence of Neck Fragmentation of ^112,124Sn + ^112,124Sn at E/A = 50 MeV" (MSU)

Two weeks after this campaign ended, LASSA was re-configured as the Si-Wall and was inside GAMMASPHERE at Argonne National Lab.  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:

• (MB-48, GSFMA-42) 141 MeV ³⁶Ar + ²⁸Si (Gammasphere, Si Wall, Microball, Neutron detectors, D. Rudolph, D.G. Sarantites et al.) (MB-48A, GSFMA-42A) 12 MeV p + ¹²C (Si Wall and Microball, stand-alone, Washington U. group). Completed successfully. The calibration of the Si Wall with protons worked very well. The a-particle calibration of the DE detectors showed superb energy resolution (better than 40 keV FWHM at 5.8 MeV).

  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 6° - 44°  forward angles.

Following its trip to ANL, LASSA returned to MSU-NSCL in January for an experiment to study the triple differential cross-section maps and their association with the PLF velocity in Xe + Sn and Au reactions at 40 MeV (Sobotka).

For comments and corrections send mail to: desouza@indiana.edu