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PH 122 (46)
Lecture

# Radiation effects, using radiation to treat, PET scan, energy balance in the Earth system

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School
Department
Physics
Course
PH 122
Professor
Brian Jones
Semester
Spring

Description
14 April 238 1 239 92U + 0 → 92 What is the likely decay mode? Just gained a neutron, therefore neutron rich β minus decay 239 239 0 −¿ 92 → 94Pu + −1e ¿ Warming Up A 60 kg laboratory worker receives a whole-body x-ray exposure of 0.50 mSv. The x-ray wavelength is 0.15 nm. How many x-ray photons are absorbed in the worker’s body? (X rays have an RBE of 1.) 1240 1240 -15 E photon λ = 0.15nm = 1.3 x 10 J Dose: 0.5 mGy RBE is 1) 0.5 mGy = 0.5 x 10 J/kg 0.5 x 10 J/kg x 60 kg = 0.03 J absorbed 0.03J 13 1.3x10 J/photon = 2.3 x 10 photons Radiation Effects The effects of radiation on tissue in the body depend on the susceptibility. Rapidly- dividing tissues are the most susceptible Tumors are extremely susceptible. Bone marrow is very susceptible. The lining of the digestive tract is quite susceptible. The brain is not very susceptible. Damaging bone marrow: 2 Sv Damaging the brain: 50 Sv Radiation can cause or cure cancer. Using Radiation to Treat Irradiating tumors Collimator allows gamma rays to penetrate only along certain lines The paths of the allowed gamma rays intersect at the tumor Radioactive “seeds” 125 125 The I in the seeds decays to Te, which emits low-energy x rays and gamma rays. Gamma scans “Hot spots” mean the isotope is present. The scan shows metabolic activity, not the bone structure. It shows areas where fractures have occurred. SPECT scan – single photon emission computed tomography PET Scan Positron emission tomography The positron annihilates with an electron to produce two oppositely directed gamma rays. Coincidence detection of two gamma rays means that the positron source is along that line. Creating positrons for use 1
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