Apparatus Designed and Built by Physicists who have taught in the Advanced Undergraduate Lab
AntiMatter Matters
TeachSpin introduces our new experiment ‘AntiMatter Matters’,
connecting on the one hand to the quantum field theory of antiparticles,
and on the other hand to the medical-physics application of Positron Emission Tomography.
Introduction
AntiMatter Matters is the place for students to see the table-top consequences of the creation and annihilation of positrons, the anti-particle companion of electrons. We offer a system that allows the detailed study of the pairs of gamma rays that emerge from the site of such annihilations, in a context that allows students to understand why this makes possible the medical technique of the PET-scan.
We have built into this experiment a list of highly desirable features, including
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Table-top operation with license-exempt radiation sources
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Full operational control of gamma-ray detection and event processing
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Convincing demonstration and application of time-coincidence techniques
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Gamma detection and energy measurement by CsI(Tl)/SiPM detectors
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Genuine source-localization, exploiting positron-annihilation gamma-ray pairs
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Full electronics, requiring only an oscilloscope – multi-channel analyzer not needed
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Experimental arena with all the tools needed for experimental layouts
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Additional projects including active-target Compton effect, g-g correlation, and photoproduction of positrons
Instrument
We have aimed to make our AntiMatter Matters kit fully equipped, by including
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An arena or experimental table for all the equipment, of footprint 64 ´ 48 cm
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An electronic controller, for powering the gamma-ray detectors, and for all the pulse-processing needed to operate the time-gated 3-channel event counter
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All cables, cords, and adapters required
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Three gamma-ray sources, 10 mCi each of Na-22 and Cs-137, and 1 mCi of Co-60
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Two CsI(Tl)+SiPM gamma detectors, (14 ´ 14 ´ 25 mm3), fully mounted
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Magnetic bases and posts for detectors, and holders for sources, attenuators, and scattering targets
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Round table with cover for ‘phantoms’ of sources hidden-from-view
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Pb-lined ‘howitzer’ for collimation for sources, and various Pb wedges and blocks for shielding
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Protractor and radial links for angular variation
Users of AntiMatter Matters will need to supply a generic digital oscilloscope; users may, but do not need to, deploy a multichannel analyzer
Experiments
This experiment depends on the availability of positrons spontaneously emitted in the decay of Sodium-22, an available isotope of 2.6-year half-life. Such positrons don’t go far; most annihilate withing the plastic disc of the sealed (10-mCi) source. But nearly all such annihilations result in a highly distinctive ‘fingerprint’, namely two g-rays each of energy 511 keV, that emerge simultaneously in time, and back-to-back in momentum.
Then the use of two energy-sensitive gamma-ray detectors enables the detection of such events, first as electrical pulses from the CsI(Tl) + SiPM detectors, and then the pulse-processing of such events in the electronic controller that’s part of the AMM experiment. Students will learn to configure those detectors and the electronics until they get a stream of pulse events in which both detectors see the deposition of a full 511 keV, in time-simultaneous events. Count rates of such gold-plated annihilation-created events can be > 50 per second, while the background count rate of events ‘accidentally’ meeting the criteria is under 0.2/s.
Students can also easily confirm that such events are only visible when the positron source lies in space along the line joining the two detectors, and can immediately understand why this has to be so. And that is enough to put an imaging modality into their hands; we provide a cute little enclosure in which a positron-emitting (and also some other gamma-emitting) sources can be hidden, and they can locate that source despite its hiding under an opaque cover.
With a total of three radioactive sources, and two energy-sensitive gamma-ray detectors, there are plenty of experiments, other than positron annihilation, that can be addressed using the tools in the AntiMatters package. We’ve had the pleasure of seeing g-g coincidences in Cobalt-60 gamma ray pairs, and seeing the Compton effect using the Cesium-137 source and one of the detectors as an ‘active target’. And of course there are introductory experiments in the inverse-square law, and in exponential absorption laws, that are easily and accurately performed using these tools.
Results
Here are some of the many results that students can capture with AMM tools.
First, with one gamma-detector powered (by the +29-V supplies within the AMM electronics) and illuminated by the 661-keV g-rays from Cs-137, they can immediately see the distribution of pulses emerging from the scintillator + optical detector:
Here it’s easy to see the concentration of pulse heights near 4.0 V, attributable to the deposition of a full 662 keV of energy in the scintillator; there are also lower-height pulses, due to partial deposition.
Next, upon activating the lower, and upper, voltage action-levels in the single-channel-analyzer (SCA) section of the electronics, users can get a logic pulse (at upper right) that occurs only for analog pulse-heights in a chosen range. Via ‘scope triggering on that logic pulse, the image below shows (at lower left) only those pulses ‘in the photopeak’, i.e. pulses in which the deposited energy is neither too small nor too large.
Similarly, illuminating a single detector with the g-rays from a Na-22 source, users can see the full pulse-height distribution (left panel below) or can arrange the SCA to give a logic-pulse output only for full-energy-deposition events (right panel) attributable to 511-keV gamma rays.
Now with two detectors deployed, both of them SCA-equipped to recognize full-energy-deposition events at 511 keV, users can next connect the Coincidence module to give new logic pulses that only appear for pairs of input-events that are simultaneous to <0.1 ms tolerance. Then the Counter section of the electronics can be used to display, during a common (say) 10-s counting interval, the number of channel-A events, the number of channel-B events, and the number of these coincidence-events.
This non-trivial rate of coincidence detections drops toward accidental-coincidence levels if the Na-22 source is not on the geometric line joining the two detectors, or if the coincidence-timing circuitry is adjusted to look for events of non-zero time separation. Here are the 10-s count totals observed when the Na-22 source is moved laterally by just 1 cm:
In this data, the observed ‘accidental coincidence rate’ is near 0.6/s; the expected rate of purely-accidental coincidences (given the observed rate of ch.-A and ch.-B events) is near 0.1 /s.
Now for orientation, here’s a cover-off view of the circular ‘stage’ onto which sources can be loaded, and hidden from view by installing an opaque cover:
Translating such a source-bearing structure laterally along one dimension, users can plot, as a function of its position, the count rates from ch.-A, from ch.-B, and from the Coincidence module. As expected, the chs. A and B rates vary only slightly with position (left panel); but the count rate of A+B coincidences (right panel) shows the narrow peak in count rate, less than 1 cm wide, that occurs when the Na-22 source passes through the line joining the two detectors.
This latter plot shows the capability of an ‘imaging modality’ based on time-coincident detection of a pair of gamma rays emerging from individual positron-annihilation events. Another such scan, under an orthogonal arrangement of the two detectors’ positions, completes the localization of the positron source on the source structure.