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.
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.