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