Radiation Detectors There are a variety of detectors that can be used to measure particles emitted from nuclear reactions. The various materials used in these detectors can be solids, liquids or gasses. The composition could contain elements spanning the periodic table from hydrogen through barium. Different detectors have varied attributes such as stopping power, energy resolution, cost and ease of handling. A combination of different detectors are often combined to create detector arrays. One material that is commonly used in detectors is silicon. To determine the energy ( E ) and location ( X,Y ) from the charge ( Q ), several equations must be used: E Front = Q Front1 + Q Front2 E Back = Q Back1 + Q Back2 E Total = E Front + E Back And: X α Q Back1 - Q Back2 Q Back1 + Q Back2 Y α Q Front1 - Q Front2 Q Front1 + Q Front2 These equations are then applied to a computer program to produce images based on what is hitting the detector. Dual-axis Duo-Lateral Position Sensitive Detectors How the DADL PSD Works Both sides of the detector are used to obtain the energy of incoming particles. Resistive Strips help to guide the charge produced by the incoming particles across the detector to the collecting edges, eliminating distortion. The gathered charge is then used to determine the energy produced by the particles as well as the particle locations. 228 Th Alpha Spectra Six very clear peaks are visible. These are the MeV, MeV, MeV, MeV, MeV and MeV. These are the six peaks that must be visible for a Silicon detector to be used. Charge Collection Guide Wires L1L1 L2L2 L2L2 L1L1 L2L2 L1L1 The detector acts much like a uniform resistor. This means the further the charge travels to reach the collection edge, the less charge. So: When L 1 = L 2, both edges receive the equal charge, or Q 1 = Q 2 When L 1 > L 2, Edge 1 receives less charge, or Q 1 < Q 2 When L 1 Q 2 Edge 1 Edge Th Alpha Position Beam Position This is a position image using a 228 Th source, which emits α - particles. These α -particles have enough energy to cause a charge to be produced as they go through the PSD. α - particles are also weak enough that they are easily handled, allowing preliminary tests to be preformed on the detector. This is a position image using the Cyclotron. A gold target was used with a silver, 15 MeV beam. Since this reaction produces higher energies, a clearer image is produced than when the 228 Th is used. Robin Dienhoffer, Texas A&M University Advisor: Dr. Sherry Yennello, Texas A&M University Silicon Detectors Crystalline silicon is very versatile as a detector material. As semiconductor, it has a band gap. The band gap can be adjusted by implanting ions into the silicon crystal. This doping results in n-type or p-type wafers which are used to create the detector. When a voltage (bias) is applied to the crystal, the band gap is enlarged thus increasing the region which is depleted in charge carriers. The depletion layer is where the particle is actually detected. The energy deposited results in a charge which is collected at the surface of the detector, resulting in a measurable signal. Over the years, a variety of approaches have been used to determine the position of a particle measured by a silicon detector. Discrete detectors require many channels of electronics to achieve good position resolution. Another avenue is resistive detectors which measure the position by charge splitting. To achieve both horizontal and vertical position, tetra-lateral detectors are the most advanced, commonly used devices. However, these result in some distortion in the signal requiring a complex algorithm for correction. Our solution to these challenges was to improve the technology available by creating a new, state of the art, detector- now known as the Dual-axis, Duo-lateral Position Sensitive Detector, or simply, a DADL PSD. F1 F2 B1 B2 Guide wires for charge Front 1 Back 2 Front 2 Back 1 Charge collecting strips