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

2. 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

3. What you need to know about how semiconductors work

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

5. 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

6. 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.

7. 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.

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

9. Detector Type Energy Conversions
Gas-filled Detector 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%

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

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

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

13. 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

14. Semiconductor Probes Often have surgical applications
Sentinel node biopsy
Parathyroid adenoma localization
Tumor localization

15.
Prekeges, J. (2010) Nuclear Medicine Instrumentation. Sudbury, MA: Jones & Bartlett. Fig 3-6 p. 32.

16. 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

17. 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

18. 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.

19. 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.

20. Next Time
Factors Relating to Radiation Detection