A large statistics study of the performance and yields of generation-6 VLPCs (HISTE-VI) Page: 1 of 4
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FERMILAB-Conf-99/383-E DO June 2000
A large statistics study of the performance and yields of generation-6 VLPCs (HISTE-VI)
D. Lincoln (for the DO Collaboration, Central Fiber Tracker Group)
Fermi National Accelerator Laboratory, P.O Box 500, Batavia, IL 60510, USA
In this paper, the DO collaboration reports on the test results of 142 880 VLPC pixels. We have explored
the space of operating conditions and find good performance at a temperature of 9 K and an average bias
voltage of 7 V. Preliminary tests have shown an average quantum efficiency of 80% and a gain of 20 000 -
60 000. Tests have shown that the devices can be made to work at very high rates. The pixel-to-pixel
variation within a chip is manageable, allowing one to assemble like-performing VLPC chips into cassettes
containing 1024 pixels each.
The DO experiment is currently embarked on
an ambitious upgrade  of its central tracking
system. The earlier tracker has been removed and
is being replaced by a super-conducting solenoid
magnet, a large silicon vertex detector and an
innovative tracker consisting of scintillating
fibers. Each scintillating fiber is read out by an
individual Visible Light Photon Counter (VLPC).
These devices are solid-state photodetectors
consisting of layers of doped and undoped silicon
. They have been previously used in a much
smaller context . In the HISTE-VI model, each
VLPC pixel has a 1mm diameter circular shape,
well matched for our scintillating fiber. The
pixels are arranged in a 2 x 4 array on a chip.
These chips are then soldered to an aluminum
nitride substrate. The chip is connected to pads
on the substrate with gold wire. These pads allow
electrical access to the VLPCs. VLPC chips are
epitaxially grown on 3.5" diameter wafers (176
chips/wafer). Twelve different loadings of the
reactor, each containing 21 wafers, were needed
to provide enough VLPCs for DO.
Earlier publications  have described the
general properties and operating conditions of
VLPCs and the techniques used to test them. This
proceeding is intended to update the results
presented in these earlier publications. The data
set presented here comprises 17860 tested
hybrids, with 15825 successfully passing. We are
currently reworking and retesting the failed chips,
with a recovery rate on the order of 50%.
Fig. 1 shows the signal distribution of a VLPC
pixel when operating with no light illuminating it
and also with a low level of illumination. The
individual photoelectron (pe) peaks are clearly
evident. When no light is present, one
occasionally observes a single electron in the gate
of the ADC, generated by thermal processes.
When VLPCs are run at temperatures above their
nominal operating point (6-12 K, depending on
operating voltage), the rate of thermal electrons
(or internally generated noise) is much increased
and the single pe peak becomes more pronounced.
1 pe (truncated)
1 3 pe
Figure 1 Typical signal distributions with and
without an active light source.
The test suite for VLPCs is described in Ref. 4,
but the basic idea is that we choose an operating
temperature that gives good gain and quantum
efficiency characteristics, while minimizing
thermal noise. This temperature was 9 K. We
then tested the VLPCs by illuminating them with
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Lincoln, Don. A large statistics study of the performance and yields of generation-6 VLPCs (HISTE-VI), article, June 23, 2000; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc712024/m1/1/: accessed November 12, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.