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Facilities
In
using PALS with thin films, an electrostatically or magnetically focused
beam of several keV positrons is generated in a high vacuum system using a
radioactive beta-decay source. Two electrostatically focused positron beam systems are schematically shown
below. In
both systems, a 25 mCi 22Na radioactive source is used to
produce positrons. he beam energy
can be varied between 0.25 through 6 keV in System I and up to 20 keV in
System II. The final beam
spot size is on the order of 1 mm diameter in System I and about 5 mm in
System II.
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SYSTEM I: Depth-Profiled Positron Spectrometer
| Specifications |
|---|
| Base Pressure |
10-10 Torr |
| Source |
50 mCi of 22Na |
| Positron Rate on Target |
100,000 per second |
| Surface Characterization Tools |
LEED, AUGER |
| Positron Spectroscopies Available |
DBS, RPS, PALS |
| Positron Implantation Energy |
250 - 25,000 eV |
| Positron Beam Diameter |
1 mm |
The Depth-Profiled Positron Spectrometer can best be envisioned as a standard
surface analysis chamber with a variable energy positron beam attached. This
allows the characterization of samples with standard surface science techniques
in coordination with the application of positrons. The positron beam
originates at the source end where a 50 mCi 22Na source emits
positrons into a thin nickel moderating foil. The moderated positrons are then
focussed up a vertical section and bent ninety degrees at the top by an
electrostatic plane mirror. This prevents the sample from seeing non-moderated
high-energy positrons. In the horizontal section, the beam is accelerated
anywhere from 0 to 5000 eV using a Heddle accelerator and finally focussed into
a small spot onto the target. The target can be biased from ground down to
negative 20,000 volts providing for an ultimate implantation energy of around
25,000 eV and the ability to do depth-profiling.
Click for a history of the Depth-Profiled Positron Spectrometer
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SYSTEM
II: Dedicated High-Rate PALS Beam
Due the the remarkable success of our Low-K
Dielectric Thin Film Research we have built a new high-rate
positron beam dedicated to beam-PALS studies of low-k thin films
and polymer thin films. Since such films do not require
particularly high vacuum, we had a lot of flexibility in its
design.
A key design goal was load-lock capability. In the current beam
we use for this research, it takes an entire day to install new
samples and pump down to the point where we are ready to collect
data. If we do not want to heat the samples, we can include four
samples at a time. If we want to heat, we can only include one
sample at a time. The reason for the long pumpdown time is that
we must bring the entire system up to air each time we change
samples. In the new beam, we only need to bring the load-lock chamber up
to air and most of the rest of the system can be kept
under vacuum during sample swaps. A few tests have demonstrated
that our pumpdown time will be reduced from one day to twenty
minutes.
To the right is a rendered view of the new beam and its internal
optics. An input arm forms an initial slow positron beam that is directed
into a
cylindrical mirror analyzer which bends the slow positrons out of
the beam of fast positrons and into the output arm. The positrons
are directed onto a target in the target chamber where secondary
electrons are knocked out providing the source of our START
signal. The stop signal comes from detecting one of the
annihilation gamma rays from two PMT/fast-plastic scintillator
detectors that are inserted in two reentrant flanges on either
side of the target chamber.
A major design improvement from our current beam is the
use of a conical anode in the CEMA assembly we use to detect the
secondary electrons to get our START signal. We flare the inner
and outer conductors in such a way as to maintain the 50 ohm
impedence of our coaxial cables from the electronics all the way
into the vacuum system and right up to the rear CEMA plate. This
results in much less "cable bounce" and our START signal is quite
clean. On the left is a rendered view of the conical anode timing
assembly exploded to show all of its parts.
Preliminary tests show that we can expect better timing than the
current system and about twenty times the signal rate. A spectrum
that used to take several hours in the current beam will take tens
of minutes to collect with the new beam. This coupled with the
rapid sample changing capability of the load-lock system results in a
massive improvement.
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