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(U)
off-the-shelf components whenever
possible.
In
particular, we
chose
a Hamamatsu
silicon
microstrip radiation detector as the key element.
Although charge-sensitive preamplifiers are
usually selected when using a semiconductor detector, such preamps are awkward to use in large
numbers.
Because we required 48 active strips, a compact LeCroy current-sensitive preamplifier
and discriminator unit (2735B) was purchased.
The 2735B cards were originally designed for
use with wire chamber detectors but we were assured by the manufacturers that the cards could
be mated with a semiconductor device.
As we have learned,
a substantial development effort
was required to reduce the noise of the system and create an interface between the detector and
the 2735B.
The balance
of
this
note
describes the work which was necessary
to
eliminate
sufficient noise to observe the alpha particles.
a.
(U)
Initial System
(1)
(U)
Connector Noise
(U) The
discriminator
card
is
a
current-sensitive
preamplifier
that
allows the noise floor to be adjusted using the threshold control.
The system uses this threshold
control to calibrate the magnitude
of the current pulses.
One
volt on the threshold
line
will
discriminate against
2 pA of signal.
If
all 48 channels are high until the threshold voltage
is
increased to 10
volts, then the noise
floor will be 20 pA.
As was determined later, the alpha
particles produce 40 pA pulses, not visible in the original system which exhibited 48 pA of noise.
The discriminator was on the outside of the vacuum chamber and was connected to the
strip
detector by twisted-pair ribbon
cables and two vacuum
feedthroughs.
The contacts on the
feedthrough connectors contributed 10 pA of noise each.
Because there was a connector on
each
side
of the
feedthroughs,
the
noise contribution was 20 pA.
This was determined by
unplugging the connectors on each side of the feedthroughs, one
at a time, and watching the
threshold voltage go down 5 volts per connector (10 pA).
This noise was reduced by relocating
the discriminator cards inside the vacuum chamber and eliminating the feedthroughs in this part
of the circuit.
Now, only logic signals pass through the feedthroughs in the base plate, not the
low-level,
current-sensitive
lines.
(2)
(U) Hybrid Preamplifiers
(U) The wiring between the strip detector and the 2735B was originally
coaxial cables below the base plate and twisted pair ribbon cable inside the chamber.
To reduce
the noise, the twisted pair was replaced by coax inside the chamber.
When
it became evident
that the wire was not the problem but rather that connectors were making the noise, we decided
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that the 2735B cards had to be placed inside the chamber. We resisted doing this
initially for
several reasons:
•
The 2735B cards may be damaged in a
high vacuum,
•
Overheating may occur.
•
The system may be contaminated through outgassing.
(U) The heart of the 2735B
is
four custom hybrids called HILs.
We were
concerned that these sealed hybrids would not work under high vacuum, but LeCroy guaranteed
us that they could even be used in deep space.
The HILs require 1.5 amps at -5 volts and thus
produce a great deal of heat.
Without convection cooling in the vacuum chamber, overheating
could make long runs impossible.
To solve this, we used the strip-detector mounting plate as a
heat sink and mounted the 2735B cards on
it backwards, with the HILs sandwiched in between
the PC board and the plate. A leaf of indium foil was inserted between the HIL and the plate to
provide
better heat conduction.
Using the mounting plate
as
a heat
sink, we
stabilized the
temperature of the HILs to 50 °C.
Our last reservation about mounting the HILs in a vacuum
was our concern for outgassing.
This would lower our vacuum pressure and distort the path of
the alpha particles.
However, no outgassing has yet been detected.
b.
(U)
Pulse Processing
(U) The 2735B performs to its specifications and the system noise
is only 2 pA
with no input.
Our next task was to find out why the alpha particles were not visible at this time.
Detailed analysis of the detector preamplifier circuit revealed a subtle problem not addressed by
any vendors.
(U) We have shown the bulk silicon of the device to be N-type, as
it
is in the
'
Hamamatsu detector. When such a device
is reverse-biased, a positive dc voltage
is applied to
the highly doped N+ contact. An alpha particle that enters the depletion volume (the bulk of the
silicon)
will generate a cloud of holes
(positive charges) and electrons
(negative charges)
in
its
path.
Fundamental semiconductor
physics
dictates
that
the
holes
will be
collected
at
the
negative-biased contact and the electrons
will collect at the positive-biased contact.
(U)
In our microstrip detector, the fabrication of the device determined that
we bias the entire unit through the N+ substrate and make individual connection to the 48 signal
inputs from the P+ contacts.
Therefore, the 2735B input signal was in effect biased negatively
with respect to the substrate.
However, as described above, the holes
(i.e., positive charge) are
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collected at that contact.
As a consequence, a positive current pulse was being transmitted to the
input of the 2735B.
(U)
LeCroy’s specifications state that the 2735B card
is designed for negative
current pulses and
will reject opposite polarity pulses up to 50
|j.A.
(U)
Using
a
single-channel
charge-sensitive
preamplifier
and
a
nuclear
spectroscopy shaping
amplifier, we were
able
to
carefully examine
the alpha
particle
pulses
generated by the microstrip detector. We determined that the charge pulse should be equivalent
to a 40 pA positive current pulse.
Given the rejection characteristics of the 2735B,
it was clear
that our positive alpha particle pulse would not be detected.
(U) We elected to design a pulse-inverting
circuit.
Two custom PC boards
with 24 pulse-inverting transformers were made and installed on the output of the strip detector.
Because the current pulse’s duration
is 10 ns, we selected an RF pulse transformer so the signal
would not be attenuated.
(U) These
transformers
have
an
output
impedance
of
ohms,
which
effectively short-circuited the input of the 2735B.
It was necessary to add a 0.1-pF ac coupling
capacitor to the circuit.
At this point, the alpha particles became visible as a normal distribution
on the computer screen for the
first time.
c.
(U)
Present System Performance
(U)
After these modifications were made to the system,
the noise
floor was
brought down to 24 pA and discriminated away.
Only signals larger than 24 pA will be seen by
the computer. The system can now see the alpha particles in real time, and is sensitive enough to
use as a tool to see any system noise.
The noise has been greatly reduced; however, noiseless
performance outside of the alpha particle beam has not yet been attained.
Two types of noise
have been found and need to be eliminated for an infinite signal-to-noise ratio.
These types are
as follows:
•
Spurious,
intermittent, random noise.
•
Parallel noise pulses.
(1)
(U) Random Noise
(U) There
is
a
rare,
random
event
that
shows up
intermittently
in
random places.
In the data presented at the end of this paper, the alpha particles are very visible
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in the middle with a few single noise pulses randomly dispersed.
This random noise
is produced
by defects in the
silicon
strip detector.
Detectors can be specially selected
for their low noise
characteristics and a new detector could be purchased to eliminate these sporadic,
individual
noise pulses.
(2)
(U)
Parallel Noise
1.
DC
Line Noise
(U) Another
anomaly
in
the
system
is
a
dc
noise
pulse
that
intermittently pulses every, channel simultaneously.
This
is referred to as a parallel noise pulse
and
it can be caused only by an event that affects all 48 channels in parallel.
The discriminator
power
supplies,
threshold
voltage
supply, and
the
strip
detector
high
voltage
supply
are
all
connected to the 48 lines and could be a common source of noise. A noise pulse on one of the
voltage lines could induce this type of noise pattern.
Such noise
is found when the telephone
is
used.
If the telephone is lifted off the hook,
it induces several parallel noise pulses in the system
and increases the count on each channel. A ringing telephone will not affect the experiment but,
a? a precaution, the telephones were forwarded whenever data was collected.
2.
AC Line Noise
(U) The power line was the next suspect point for ac line noise getting
into the system.
All electronic equipment was plugged into a single power line filter/conditioner,
including the computer, CAMAC crate, and the discriminator power supply. When overloading
the conditioner reduced
its effectiveness, the computer and CAMAC crate were removed from
the conditioner and plugged into the wall. The discriminator power supply and voltage threshold
supply were
left on the power-line
conditioner
to reduce any
noise
to
the
charge-sensitive
electronics. To help filter low frequency noise on the high-voltage bias line of the strip detector,
an RC
filter was used with a time constant of 100 ms.
d.
(U)
Electro-Magnetic Noise
(U) The
sensitive
inputs
of
the
discriminator
make
it
vulnerable
to
electro-magnetic pickup.
The presence of a large electro-magnetic field could account for the
induction
of
a
parallel
noise
pulse
being
induced
in
the
output
wire
that connects
to
the
discriminator input and acts as a receiving antenna.
Shielded coaxial cables were used at
first,
but their capacitance affected the charge-sensitive inputs of the discriminator.
Shielding the bell
jar of the vacuum system, where
all the charge sensitive electronics are held, helped shield the
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