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CLRS 321Nuclear Medicine Physics &
Instrumentation I
Part C: Semiconductors and Miscellaneous Scintillation Devices
Unit II: Nuclear Medicine Measuring Devices
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– Positive charge for n side
P-side has extra holes (which you would think would leave it positively charged)
N-side has extra electrons (which you would think would leave it negatively 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 anode (+ terminal) is placed on the n-side, then this is called reverse bias and the depletion layer widens.
If the cathode (- terminal) is placed on the n-side, then this is called forward bias and the depletion layer narrows.
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 Detector25-35 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}
Prekeges, J. (2010) Nuclear Medicine Instrumentation. Sudbury, MA: Jones & Bartlett. Fig 3-6 p. 32.
http://www.breastdiseases.com/sentno.htm
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 mediumEnergy, spatial, and angular resolutionVolume sensitivityCount rate capabilities
Liquid Scintillation Detector
http://ocean.stanford.edu/lab//labo.mpe.free.fr/img/materiel/scintill.JPG
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
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.
The Plan
oComplete Homework 7 by 1:00 PM Wednesday, November 6th
oWe will review this (and grade) in lab on WednesdayoUnit II Test will be on Monday, November 11th