Malignant tumors are commonlytreated with targeted x-ray radiation therapy. To generate these medicalx rays, a linear accelerator directs a high-energy beam of electronstoward a metal target—typically tungsten. As they near the tungsten nuclei,the electrons are deflected and accelerated, emitting high-energyphotons via bremsstrahlung. The resulting x rays are collimated into abeam that is directed at the tumor. The photons can deposit energy inthe tumor through Compton and photoelectric interactions. A typicaltumor has 108 cells/cm3, and in a full treatment, 4 MeV photons mayproduce a dose of 70 Gy in 35 fractional exposures on different days.The gray (Gy) is a measure of the absorbed energy dose of radiation perunit mass of tissue: 1 Gy = 1 J/kg. While interacting with molecules (mainly water) in the tumortissue, each Compton electron or photoelectron causes a series of ionizations,each of which takes about 40 eV. Estimate the maximum numberof ionizations that one photon generated by this linear acceleratorcan produce in tissue. (a) 100; (b) 1000; (c) 10^4; (d) 10^5.
Malignant tumors are commonly
treated with targeted x-ray radiation therapy. To generate these medical
x rays, a linear accelerator directs a high-energy beam of electrons
toward a metal target—typically tungsten. As they near the tungsten nuclei,
the electrons are deflected and accelerated, emitting high-energy
photons via bremsstrahlung. The resulting x rays are collimated into a
beam that is directed at the tumor. The photons can deposit energy in
the tumor through Compton and photoelectric interactions. A typical
tumor has 108 cells/cm3, and in a full treatment, 4 MeV photons may
produce a dose of 70 Gy in 35 fractional exposures on different days.
The gray (Gy) is a measure of the absorbed energy dose of radiation per
unit mass of tissue: 1 Gy = 1 J/kg. While interacting with molecules (mainly water) in the tumor
tissue, each Compton electron or photoelectron causes a series of ionizations,
each of which takes about 40 eV. Estimate the maximum number
of ionizations that one photon generated by this linear accelerator
can produce in tissue. (a) 100; (b) 1000; (c) 10^4; (d) 10^5.
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