Commercially available ionization smoke detectors (< $10) contain a small 241Am source. We carefully removed the metal cover and mounted them in a 2x4 lego brick. These sources have been added to our approved source inventory and we treat them with the same care and training that we treat expensive thin-film sources. For the BFY demonstration, we are borrowing sources from NYU - thanks to Lorcan Folan! Do not touch the radioactive source - these alpha sources have little protective coating. These sources are not as thin as those intended for research purposes, so that some alpha particles emitted from deep within the source have lost energy leading to a broader energy spectrum than is ideal. 241Am emits alpha particles with 5.486 MeV (85% of the time), though measuring the spectrum from these smoke detector sources gives a lower energy. In most cases, experiments to measure energy loss of alpha particles are done in vacuum, or at least in a variable pressure environment. In order to have the experiment done easily by a full class of students, these measurements were done in air.
Detector:
Radiation can be detected in a number of ways: scintillation, gas ionization or in semiconductor charge detectors\cite{knoll2010}. Since alpha particles interact so strongly with matter, they penetrate only a few microns at most, so typically surface barrier detectors such as those from Ortec \cite{ortec}, which collect the charge liberated when a high energy alpha particles excited electrons and holes in a semiconductor. The liberated charge is proportional to the particle energy, so measuring this charge is a measure of alpha energy. These detectors are efficient, have low noise, large area and can measure low background regimes. For simpler measurements such as in an undergraduate experiment or for hobbyists \cite{opengeigerde} , a PIN photodiode can work as a detector. A PIN has an intrinsic semiconductor region sandwiched between heavily doped P- and N-doped regions - the large intrinsic region lowering the capacitance and decreasing the response time for optical communications. As the alpha particle enters the intrinsic region of the diode it creates electrons and holes, each swept in opposite directions by the built-in-field, creating a measurable charge. After trying several different devices, we found that the Hamamatsu S1223-01 PIN photodiode \cite{hammamatsu} works well as an alpha particle detector due to its ~10 mm2 area and low capacitance, however it doesn't seem to improve its performance with reverse biasing (unlike many such diodes). This diode has a glass window that needs to be carefully removed so that the bare silicon is exposed, and the detector must be kept in the dark. Each diode is approximately $10.
Preamplifier:
We need to amplify the small pulse of current to create a voltage pulse. Commercial charge preamplifiers are available with very high performance and low noise \cite{cremat}, however again we value price, flexibility and pedagogy over performance. We designed a simple printed circuit board with a socket for a dual op-amp that can be reconfigured for different sorts of amplifiers (voltage, current, etc.). Here we use a charge preamplifier followed by a voltage amplifier with a gain such that the full-energy peak for the alphas give a 1 V pulse. These can be reconfigured for use in projects later. We look at this as a reconfigurable amplifier - like that on a protoboard - however with performance approaching that of a commercial system. The PCB, box, connectors and other components total less than $80 each. Designs for our boards are available \cite{gillen2017}. We view this amplifier as partway between an exposed protoboard (with uncontrolled pickup, capacitances and poor connections) and an amplifier designed for a single purpose.