Semiconductor Radiation Detectors: Working Principles and Applications

Lecture 1--Semiconductors Detectors (and Miscellaneous Scintillation Devices) Unit IV: Miscellaneous Aspects of Radiation Detection CLRS 321 Nuclear Medicine Physics & Instrumentation I

Objectives • Describe n-type and p-type semiconductors and how they function as a radiation measuring device • Describe the materials and construction of a semiconductor detector • Discuss the detection and counting characteristics of a semiconductor radiation detection device and how these characteristics match up to scintillation detectors • Discuss the use of semiconductor detectors in nuclear medicine • Describe quality control measures for semiconductor detectors • Explain the function of TLD ring and collar dosimeters • Describe the function and uses of a liquid scintillation counter

What you need to know about how semiconductors work http://www.youtube.com/watch?v=PuZWoHa9mBo

Semiconduction Prekeges, J. (2010) Nuclear Medicine Instrumentation. Sudbury, MA: Jones & Bartlett. Figs 3-1 & 3-2, p. 29.

Semiconduction:p-n junction • Extra electrons (n-type) move toward the anode • Extra holes (p-type) tend to move toward cathode like +electrons • When p & n types come together, the holes and electrons diffuse to opposite ends, but end up creating an opposite “intrinsic” charge • Negative charge for p side • Because the electron acceptor impurity has lost its holes • Positive charge for n side • Because the electron donor has lost its electrons

P-side has lost its extra holes (which makes it negatively charged) N-side has lost its extra electrons (which makes it positively charged) Diffusion of holes and electrons results in the charges pictured and the intrinsic charge Prekeges, J. (2010) Nuclear Medicine Instrumentation. Sudbury, MA: Jones & Bartlett. FigB-9, p. 275.

If the cathode (- terminal) is placed on the n-side, then this is called forward bias and the depletion layer narrows. If the anode (+ terminal) is placed on the n-side, then this is called reverse bias and the depletion layer widens. With reverse bias, the depletion layer becomes a solid-state ionization chamber Prekeges, J. (2010) Nuclear Medicine Instrumentation. Sudbury, MA: Jones & Bartlett. FigB-9, p. 275.

Prekeges, J. (2010) Nuclear Medicine Instrumentation. Sudbury, MA: Jones & Bartlett. Fig 3-4 p. 31.

Detector Type Energy Conversions • Gas-filled Detector 25-35 eV to make ion pairs • Scintillation Detector 30 eV for scintillation • Semiconductor 3-5 eV to make ion pairs FWHM (662 keV Cs-137 Source) NaI(Tl): 6 to 8% Semiconductor: 1.8 to 2.5%

Comparison of Information Carriers Prekeges, J. (2010) Nuclear Medicine Instrumentation. Sudbury, MA: Jones & Bartlett. p. 33.

Prekeges, J. (2010) Nuclear Medicine Instrumentation. Sudbury, MA: Jones & Bartlett. Fig3-7, p. 34.

Seminconductor Energy Spectrum Prekeges, J. (2010) Nuclear Medicine Instrumentation. Sudbury, MA: Jones & Bartlett. Fig3-3, p. 30.

Semiconductor Materials • Cadmium (Cd), Tellurium (Te), Zinc (Zn) • ZnTe and CdTe common • “CZT” semiconductor (or detector) • Cd and Zn are electron acceptors • Have “holes” and thus are p-type • Te is an electron donor • Extra electrons and thus an n-type

Semiconductor Probes • Often have surgical applications • Sentinel node biopsy • Parathyroid adenoma localization • Tumor localization http://www.battelle.org/solutions/?Nav_Area=Solution&Nav_SectionID=9&Nav_CatID=9_DeviceDevelopment&Nav_ContentKey={74FFB370-37E6-486A-8D46-DE7E9E29715E}

http://www.breastdiseases.com/sentno.htm Prekeges, J. (2010) Nuclear Medicine Instrumentation. Sudbury, MA: Jones & Bartlett. Fig 3-6 p. 32.

Quality Control for Semiconductor Probes • Daily: • Battery Check • Background Determination • Constancy Check (using Co-57 source) • Quarterly or semiannually: • Calibration (may need to be done by manufacturer) NEMA recommendations (annually): Sensitivity in air and scattering medium Energy, spatial, and angular resolution Volume sensitivity Count rate capabilities

Liquid Scintillation Detector Usually used in laboratories to count beta emitters. Solvents dissolve radioactive samples (often purposely radiolabelled) in to vials. Radioactivity scintillates a set of solutions in the vials. PMTs detect light from the “scintillation cocktails” in the vials. No longer routinely used in nuclear medicine http://ocean.stanford.edu/lab//labo.mpe.free.fr/img/materiel/scintill.JPG

Radiation Detection (formerly known as “film”) badges Al2O3 crystalline material becomes luminescent under selected laser frequencies. Luminescence is proportional to the amount of radiation exposure.

Thermoluminescent Dosimetry (TLD Ring Badges) Uses a lithium fluoride chip that absorbs the energy of ionizing radiation. It is then heated at characteristic temperatures that cause it to emit the absorbed energy as visible light. The amount of exposure is determined by the light intensities.

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Semiconductor Radiation Detectors: Working Principles and Applications