# 10.7 Medical applications and biological effects of nuclear radiation  (Page 7/18)

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A group of scientists use carbon dating to date a piece of wood to be 3 billion years old. Why doesn’t this make sense?

According to your lab partner, a 2.00-cm-thick sodium-iodide crystal absorbs all but $10\text{%}$ of rays from a radioactive source and a 4.00-cm piece of the same material absorbs all but $5\text{%}?$ Is this result reasonable?

If $10\text{%}$ of rays are left after 2.00 cm, then only ${\left(0.100\right)}^{2}=0.01=1\text{%}$ are left after 4.00 cm. This is much smaller than your lab partner’s result ( $5\text{%}$ ).

In the science section of the newspaper, an article reports the efforts of a group of scientists to create a new nuclear reactor based on the fission of iron (Fe). Is this a good idea?

The ceramic glaze on a red-orange “Fiestaware” plate is ${\text{U}}_{2}{\text{O}}_{3}$ and contains 50.0 grams of ${}^{238}\text{U}$ , but very little ${}^{235}\text{U}$ . (a) What is the activity of the plate? (b) Calculate the total energy that will be released by the ${}^{238}\text{U}$ decay. (c) If energy is worth 12.0 cents per $\text{kW}·\text{h}$ , what is the monetary value of the energy emitted? (These brightly-colored ceramic plates went out of production some 30 years ago, but are still available as collectibles.)

a. $1.68\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-5}\phantom{\rule{0.2em}{0ex}}\text{Ci}$ ; (b) From Appendix B , the energy released per decay is 4.27 MeV, so $8.65\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{10}\phantom{\rule{0.2em}{0ex}}\text{J}$ ; (c) The monetary value of the energy is $\text{}2.9\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{3}$

Large amounts of depleted uranium $\left({}^{238}\text{U}\right)$ are available as a by-product of uranium processing for reactor fuel and weapons. Uranium is very dense and makes good counter weights for aircraft. Suppose you have a 4000-kg block of ${}^{238}\text{U}$ . (a) Find its activity. (b) How many calories per day are generated by thermalization of the decay energy? (c) Do you think you could detect this as heat? Explain.

A piece of wood from an ancient Egyptian tomb is tested for its carbon-14 activity. It is found to have an activity per gram of carbon of $A=10\phantom{\rule{0.2em}{0ex}}\text{decay/min}·\text{g}$ . What is the age of the wood?

We know that $\lambda =3.84\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-12}\phantom{\rule{0.2em}{0ex}}{\text{s}}^{\text{−}1}$ and ${A}_{0}=0.25\phantom{\rule{0.2em}{0ex}}\text{decays}\text{/}\text{s}·\text{g}=15\phantom{\rule{0.2em}{0ex}}\text{decays}\text{/}\text{min}·\text{g}.$
Thus, the age of the tomb is
$t=-\frac{1}{3.84\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-12}\phantom{\rule{0.2em}{0ex}}{\text{s}}^{\text{−}1}}\text{ln}\phantom{\rule{0.2em}{0ex}}\frac{10\phantom{\rule{0.2em}{0ex}}\text{decays}\text{/}\phantom{\rule{0.2em}{0ex}}\text{min}·\text{g}}{15\phantom{\rule{0.2em}{0ex}}\text{decays/}\phantom{\rule{0.2em}{0ex}}\text{min}·\text{g}}=1.06\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{11}\phantom{\rule{0.2em}{0ex}}\text{s}\approx 3350\phantom{\rule{0.2em}{0ex}}\text{y}.$

## Challenge problems

This problem demonstrates that the binding energy of the electron in the ground state of a hydrogen atom is much smaller than the rest mass energies of the proton and electron.

(a) Calculate the mass equivalent in u of the 13.6-eV binding energy of an electron in a hydrogen atom, and compare this with the known mass of the hydrogen atom.

(b) Subtract the known mass of the proton from the known mass of the hydrogen atom.

(c) Take the ratio of the binding energy of the electron (13.6 eV) to the energy equivalent of the electron’s mass (0.511 MeV).

(d) Discuss how your answers confirm the stated purpose of this problem.

The Galileo space probe was launched on its long journey past Venus and Earth in 1989, with an ultimate goal of Jupiter. Its power source is 11.0 kg of ${}^{238}\text{Pu},$ a by-product of nuclear weapons plutonium production. Electrical energy is generated thermoelectrically from the heat produced when the 5.59-MeV $\alpha$ particles emitted in each decay crash to a halt inside the plutonium and its shielding. The half-life of ${}^{238}\text{Pu}$ is 87.7 years.

(a) What was the original activity of the ${}^{238}\text{Pu}$ in becquerels?

(b) What power was emitted in kilowatts?

(c) What power was emitted 12.0 y after launch? You may neglect any extra energy from daughter nuclides and any losses from escaping $\gamma$ rays.

a. $6.97\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{15}\phantom{\rule{0.2em}{0ex}}\text{Bq}$ ; b. 6.24 kW; c. 5.67 kW

Find the energy emitted in the ${\beta }^{-}$ decay of ${}^{60}\text{Co}$ .

Engineers are frequently called on to inspect and, if necessary, repair equipment in nuclear power plants. Suppose that the city lights go out. After inspecting the nuclear reactor, you find a leaky pipe that leads from the steam generator to turbine chamber. (a) How do the pressure readings for the turbine chamber and steam condenser compare? (b) Why is the nuclear reactor not generating electricity?

a. Due to the leak, the pressure in the turbine chamber has dropped significantly. The pressure difference between the turbine chamber and steam condenser is now very low. b. A large pressure difference is required for steam to pass through the turbine chamber and turn the turbine.

If two nuclei are to fuse in a nuclear reaction, they must be moving fast enough so that the repulsive Coulomb force between them does not prevent them for getting within $R\approx {10}^{-14}\text{m}$ of one another. At this distance or nearer, the attractive nuclear force can overcome the Coulomb force, and the nuclei are able to fuse.

(a) Find a simple formula that can be used to estimate the minimum kinetic energy the nuclei must have if they are to fuse. To keep the calculation simple, assume the two nuclei are identical and moving toward one another with the same speed v . (b) Use this minimum kinetic energy to estimate the minimum temperature a gas of the nuclei must have before a significant number of them will undergo fusion. Calculate this minimum temperature first for hydrogen and then for helium. ( Hint: For fusion to occur, the minimum kinetic energy when the nuclei are far apart must be equal to the Coulomb potential energy when they are a distance R apart.)

For the reaction, $n+{}^{3}\text{He}\to {}^{4}\text{He}+\gamma$ , find the amount of energy transfers to ${}^{4}\text{H}\text{e}$ and $\gamma$ (on the right side of the equation). Assume the reactants are initially at rest. ( Hint: Use conservation of momentum principle.)

The energies are
$\begin{array}{cc}\hfill {E}_{\gamma }& =20.6\phantom{\rule{0.2em}{0ex}}\text{MeV}\phantom{\rule{0.2em}{0ex}}\hfill \\ \hfill {E}_{{}^{4}\text{H}\text{e}}& =5.68\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-2}\phantom{\rule{0.2em}{0ex}}\text{MeV}\hfill \end{array}$ . Notice that most of the energy goes to the $\gamma$ ray.

Engineers are frequently called on to inspect and, if necessary, repair equipment in medical hospitals. Suppose that the PET system malfunctions. After inspecting the unit, you suspect that one of the PET photon detectors is misaligned. To test your theory you position one detector at the location $\left(r,\theta ,\phi \right)=\left(1.5,45,30\right)$ relative to a radioactive test sample at the center of the patient bed. (a) If the second photon detector is properly aligned where should it be located? (b) What energy reading is expected?

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