324
8 Ionizing Radiation and Life
of energy that was deposited in the volume of a DNA molecule, taken as π(10Å) 2 ·
3.4 Å · 3 × 10 9 = 3 × 10 −18 m 3
8.18 Why are gamma rays less dangerous to biological tissue than alpha particles
if both have an energy per particle of one MeV and the same intensity?
8.19 As part of a treatment of a brain tumor, a 67.5 MeV beam of protons is used to
destroy cancerous tissue inside a volume of ten cubic millimeters. The Bragg peak
occurs at a distance from 35 to 37 mm from the entrance point. When the protons
enter the Bragg peak region, they have energies less than 1 MeV. Estimate the beam
flux in order to deposit an accumulated dose of 100 rads in 1 h.
8.20 Suppose you wish to form an image of a cancer tumor in the gut with
either ultrasonic imaging or an X-ray CT scan. Describe the relative merits and
disadvantages of each.
8.21 If beta particles of energy 606 keV from 2.4 millicurie of radioisotope Iodine131 (with half-life of 8.02 days and 89.9% emission of this β particle) initially
produce 54 ions per beta particle in 20 grams of tissue, what is the exposure (in
röentgen) after the first hour? If all the energy of the beta particles is deposited into
the 20 grams of tissue over a time of 30 days, what dose (in millirads) was given?
8 Ionizing Radiation and Life
of energy that was deposited in the volume of a DNA molecule, taken as π(10Å) 2 ·
3.4 Å · 3 × 10 9 = 3 × 10 −18 m 3
8.18 Why are gamma rays less dangerous to biological tissue than alpha particles
if both have an energy per particle of one MeV and the same intensity?
8.19 As part of a treatment of a brain tumor, a 67.5 MeV beam of protons is used to
destroy cancerous tissue inside a volume of ten cubic millimeters. The Bragg peak
occurs at a distance from 35 to 37 mm from the entrance point. When the protons
enter the Bragg peak region, they have energies less than 1 MeV. Estimate the beam
flux in order to deposit an accumulated dose of 100 rads in 1 h.
8.20 Suppose you wish to form an image of a cancer tumor in the gut with
either ultrasonic imaging or an X-ray CT scan. Describe the relative merits and
disadvantages of each.
8.21 If beta particles of energy 606 keV from 2.4 millicurie of radioisotope Iodine131 (with half-life of 8.02 days and 89.9% emission of this β particle) initially
produce 54 ions per beta particle in 20 grams of tissue, what is the exposure (in
röentgen) after the first hour? If all the energy of the beta particles is deposited into
the 20 grams of tissue over a time of 30 days, what dose (in millirads) was given?