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Fundamentals of Physics

David Halliday, Robert Resnick , Jearl Walker

Chapter 43

Energy from the Nucleus - all with Video Answers

Educators


Chapter Questions

01:40

Problem 1

The isotope $^{235} \mathrm{U}$ decays by alpha emission with a half-life of $7.0 \times 10^{8}$ y. It also decays (rarely) by spontaneous fission, and if the
alpha decay did not occur, its half-life due to spontaneous fission alone would be $3.0 \times 10^{17} \mathrm{y}$ . (a) At what rate do spontaneous fission decays occur in 1.0 $\mathrm{g}$ of $^{235} \mathrm{U} ?$ (b) How many $^{25} \mathrm{U}$ alpha-decay events are there for every spontaneous fission event?

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01:00

Problem 2

The nuclide 28 $\mathrm{Np}$ requires 4.2 $\mathrm{MeV}$ for fission. To remove a neutron from this nuclide requires an energy expenditure of 5.0
MeV.Is $^{27} \mathrm{Np}$ fissionable by thermal neutrons?

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00:58

Problem 3

A thermal neutron (with approximately zero kinetic energy) is absorbed by a 23 $\mathrm{S}$ nucleus. How much energy is transferred from
mass energy to the resulting oscillation of the nucleus? Here are some
atomic masses and the neutron mass.
$$\begin{array}{cccc}{2 \pi \mathrm{U}} & {237.048723 \mathrm{u}} & {238.050782 \mathrm{u}} \\ {230} & {239.054287 \mathrm{u}} & {240.056585 \mathrm{u}} \\ {\mathrm{n}} & {1.008664 \mathrm{u}}\end{array}$$

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01:49

Problem 4

The fission properties of the plutonium isotope $^{299} \mathrm{Pu}$ are very similar to those of $^{\frac{13}{25}} \mathrm{U}$ The average energy released per fission is 180 $\mathrm{MeV}$ . How much energy, in MeV, is released if all the atoms in
1.00 $\mathrm{kg}$ of pure 33 $\mathrm{Pu}$ undergo fission?

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01:57

Problem 5

During the Cold War, the Premier of the Soviet Union threatened the United States with 2.0 megaton 23 $\mathrm{Pu}$ warheads. (Each would have yielded the equivalent of an explosion of 2.0 megatons
of TNT, where 1 megaton of TNT releases $2.6 \times 10^{28}$ MeV of energy.) If the plutonium that actually fissioned had been 8.00$\%$ of the total mass of the plutonium in such a warhead, what was that total mass?

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06:33

Problem 6

(a) - (d) Complete the following table, which refers to the generalized fission reaction $^{28} \mathrm{U}+\mathrm{n} \rightarrow \mathrm{X}+\mathrm{Y}+b \mathrm{n}$

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00:37

Problem 7

At what rate must $^{255} \mathrm{U}$ nuclei undergo fission by neutron bombardment to generate energy at the rate of 1.0 $\mathrm{W}$ ? Assume
that $Q=200 \mathrm{MeV} .$

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02:25

Problem 8

(a) Calculate the disintegration energy $Q$ for the fission of the molybdenum isotope 98 Mo into two equal parts. The masses you will need are 97.90541 $\mathrm{u}$ for $^{98} \mathrm{Mo}$ and 48.95002 $\mathrm{u}$ for $^{49} \mathrm{Sc} .$ (b) If $Q$ turns out to be positive, discuss why this process does not occur spontaneously.

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02:34

Problem 9

(a) How many atoms are contained in 1.0 $\mathrm{kg}$ of pure $^{235} \mathrm{U} ?$ (b) How much energy, in joules, is released by the complete fissioning of 1.0 $\mathrm{kg}$ of $^{2 \mathrm{ss}} \mathrm{U}$ ? Assume $Q=200 \mathrm{MeV}$ . (c) For how long would
this energy light a 100 $\mathrm{W}$ lamp?

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02:28

Problem 10

Calculate the energy released in the fission reaction $$
^{228} \mathrm{U}+\mathrm{n} \rightarrow^{141} \mathrm{Cs}+^{\mathrm{a}} \mathrm{Rb}+2 \mathrm{n}$$
Here are some atomic and particle masses.
$$\begin{array}{llll}{235 \mathrm{U}} & {235.04392 \mathrm{u}} & {\text { "Rb }} & {92.92157 \mathrm{u}}\end{array}$$
$$\begin{array}{lll}{141 \mathrm{Cs}} & {140.91963 \mathrm{u}} & {\mathrm{n}} & {1.00866 \mathrm{u}}\end{array)$$

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00:54

Problem 11

Calculate the disintegration energy $Q$ for the fission of "Crinto two equal fragments. The masses you will need are
$$^{s 2} \mathrm{Cr} \quad 51.94051 \mathrm{u} \quad 2 \mathrm{Mg} \quad 25.98259 \mathrm{u}$$

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02:43

Problem 12

Consider the fission of 23 $\mathrm{U}$ by fast neutrons. In one fission event, no neutrons are emitted and the final stable end
products, after the beta decay of the primary fission fragments, are
tha Ce and "Ru. (a) What is the total of the beta-decay events in the
two beta-decay chains? (b) Calculate $Q$ for this fission process.
The relevant atomic and particle masses are

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02:56

Problem 12

Consider the fission of 23 $\mathrm{U}$ by fast neutrons. In one fission event, no neutrons are emitted and the final stable end
products, after the beta decay of the primary fission fragments, are
tha Ce and "Ru. (a) What is the total of the beta-decay events in the
two beta-decay chains? (b) Calculate $Q$ for this fission process.
The relevant atomic and particle masses are
$$238.05079 \mathrm{u} \quad 14 \mathrm{Ce} \quad 139.90543 \mathrm{u}$$

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02:06

Problem 13

Assume that immediately after the fission of $^{236} \mathrm{U}$ according to Eq. $43-1$ , the resulting
140 $\mathrm{X}$ and $$94 \mathrm{S}$$ nuclei are just touching $
\begin{array}{l}{\text { at their surfaces. (a) Assuming the nuclei to be spherical, calculate }} \\ {\text { the electric potential energy associated with the repulsion between }} \\ {\text { the two fragments. (Hint: Use Eq. } 42-3 \text { to calculate the radii of the }} \\ {\text { fragments.) (b) Compare this energy with the energy released in a }} \\ {\text { typical fission event. }}\end{array}$

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04:21

Problem 14

$\mathrm{A}^{266} \mathrm{U}$ nucleus undergoes fission and breaks into two middle-mass fragments, 140 $\mathrm{Xe}$ and 9 $\mathrm{Sr}$ . (a) By what percentage does the surface area of the fission products differ from that of the original $^{266} \mathrm{U}$ nucleus? (b) By what percentage does the volume change? (c) By what percentage does the electric potential energy change?
The electric potential energy of a uniformly charged sphere of ra-
dius $r$ and charge $Q$ is given by $$U=\frac{3}{5}\left(\frac{Q^{2}}{4 \pi \varepsilon_{0} r}\right)$$

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03:05

Problem 15

A 66 kiloton atomic bomb is fueled with pure $^{225} \mathrm{U}$ (Fig. $43-14 ), 4.0 \%$ of which actually undergoes fission. (a) What is the
mass of the uranium in the bomb? (It is not 66 kilotons - that is
the amount of released energy specified in terms of the mass of TNT required to produce the same amount of energy.) (b) How
many primary fission fragments are produced? (c) How many fission neutrons generated are released to the environment? (On average, each fission produces 2.5 neutrons)Figure $43-14$ Problem $15 . \mathrm{A}$ "button" of 23 $\mathrm{U}$ ready to be recast and
machined for a warhead.

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02:45

Problem 16

In an atomic bomb, energy release is due to the uncontrolled fission of plutonium $$
^{239} \mathrm{Pu}\left(\text { or }^{235} \mathrm{U}\right)$$ nitude of the released energy, specified in terms of the mass of TNT required to produce the same energy release. One megaton
of TNT releases $2.6 \times 10^{28}$ MeV of energy.(a) Calculate the rating.in tons of TNT, of an atomic bomb containing 95.0 $\mathrm{kg}$ of $^{239} \mathrm{Pu},$ of which 2.5 $\mathrm{kg}$ actually undergoes fission. (See Problem $4 . )$ (b) Why is the other 92.5 $\mathrm{kg}$ of $^{239} \mathrm{Pu}$ needed if it does not fission?

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05:57

Problem 17

In a particular fission event in which $^{23} \mathrm{U}$ is fissioned by slow neutrons, no neutron is emitted and one of the primary fission fragments is $^{3}$ Ge. (a) What is the other fragment? The disintegration energy is $O=170 \mathrm{MeV} .$ How much of this energy goes to (b) the $^{83}$ Ge fragment and (c) the other fragment? Just after the fission, what is the speed of (d) the "Ge fragment and (e)the other fragment?

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03:43

Problem 18

A 200 $\mathrm{MW}$ fission reactor consumes half its fuel in 3.00 $\mathrm{y}$ .
How much $^{235} \mathrm{U}$ did it contain initially? generated arises from the fission of 235 $\mathrm{U}$ and that this nuclide is consumed only by the fission process.

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01:07

Problem 19

The neutron generation time $t_{\mathrm{gee}}$ in a reactor is the average time needed for a fast neutron emitted in one fission event to be
slowed to thermal energies by the moderator and then initiate an-
other fission event. Suppose the power output of a reactor at time $t=0$ is $P_{0}$ . Show that the power output a time $t$ later is $P(t),$ where $P(t)=P_{0} k^{n} \sec$ and $k$ is the multiplication factor. For constant power output, $k=1$

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02:16

Problem 20

A reactor operates at 400 $\mathrm{MW}$ with a neutron generation time (see Problem 19) of 30.0 $\mathrm{ms}$ . If its power increases for 5.00
min with a multiplication factor of $1.0003,$ what is the power output
at the end of the 5.00 $\mathrm{min}$ ?

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01:50

Problem 21

The thermal energy generated when radiation from radionuclides is absorbed in mattliter can serve as the basis for a small power
source for use in satellites, remote weather stations, and other isolated locations Such radionuclides are manufact in abundance in nuclear reactors and may be separated chemically from the spent
$$^{123 \mathrm{Pu}}\left(T_{1 / 2}=87.7 \mathrm{y}\right),$$
alpha emitter with $Q=5.50 \mathrm{MeV}$ . At what rate is thermal energy
generated in 1.00 $\mathrm{kg}$ of this material?

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01:44

Problem 22

The neutron generation time $t_{\text { gen }}$ (see Problem 19$)$ in a particular reactor is 1.0 $\mathrm{ms}$ If the reactor is operating at a power level of 500 $\mathrm{MW}$ , about how many free neutrons are present in the reactor at any moment?

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01:34

Problem 23

The neutron generation time (sce Problem 19$)$ of a particular reactor is 1.3 $\mathrm{ms}$ . The reactor is generating energy at
the rate of 1200.0 $\mathrm{MW}$ . To perform certain maintenance checks, the power level must temporarily be reduced to 350.00 $\mathrm{MW}$ . It is desired that the transition to the reduced power level take 2.6000 $\mathrm{s}$ . To what (constant) value should the multiplication factor be set to
effect the transition in the desired time?

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03:26

Problem 24

(See Problem $21 .$ ) Among the many fission products that may be extracted chemically from the spent fuel of a nuclear reactor is $^{2} \mathrm{Sr}\left(T_{1 / 2}=29 \mathrm{y}\right) .$ This isotope is produced in typical large reactors at the rate of about 18 $\mathrm{kg} / \mathrm{y}$ . By its radioactivity, the isotope
generates thermal energy at the rate of 0.93 $\mathrm{W} / \mathrm{g}$ (a) Calculate the effective disintegration energy $Q_{\text { erf }}$ associated with the decay of a 9. Sr nucleus. (This energy $Q_{\text { ent }}$ includes contributions from the decay of the "Sr daughter products in its decay chain but not from neutrinos, which escape totally from the sample.) (b) It is desired
to construct a power source generating 150 $\mathrm{W}($ electric power) to
use in operating electronic equipment in an underwater acoustic beacon. If the power source is based on the thermal energy generated by 90 $\mathrm{Sr}$ and if the efficiency of the thermal-electric conversion process is $5.0 \%,$ how much $^{90} \mathrm{Sr}$ is needed?

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04:16

Problem 25

(a) A neutron of mass $m_{\mathrm{n}}$ and kinetic energy $K$ makes a head-on clastic collision with a stationary atom of mass $m$ . Show that the fractional kinetic energy loss of the neutron is given by
$$\frac{\Delta K}{K}=\frac{4 m_{\mathrm{n}} m}{\left(m+m_{\mathrm{n}}\right)^{2}} $$
$$\Delta K / K$$
$$\begin{array}{l}{\text { (b) hydrogen, (c) deuterium, (d) carbon, and (e) lead. (f) If }} \\ {K=1.00 \text { MeV initially, how many such head-on collisions would it }} \\ {\text { take to reduce the neutron's kinetic energy to a thermal value }} \\ {(0.025 \mathrm{eV}) \text { if the stationary atoms it collides with are deuterium, a }} \\ {\text { commonly used moderator? (In actual moderators, most collisions }} \\ {\text { are not head-on.) }}\end{array}$$

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02:40

Problem 26

How long ago was the ratio $$^{235} \mathrm{U} /^{288} \mathrm{U}$$ in natural uranium deposits equal to 0.15$?$

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01:28

Problem 27

The natural fission reactor discussed in Module $43-3$ is estimated to have generated 15 gigawatt-years of energy during its
lifetime. (a) If the reactor lasted for $200000 \mathrm{y},$ at what average
power level did it operate? (b) How many kilograms of $^{255} \mathrm{U}$ did it
consume during its lifetime?

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01:03

Problem 28

Some uranium samples from the natural reactor site described in Module $43-3$ were found to be slightly enriched in 25 $\mathrm{U}$ ,
rather than depleted. Account for this in terms of neutron absorp-
tion by the abundant isotope 28 $\mathrm{U}$ and the subsequent beta and
alpha decay of its products.

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03:21

Problem 29

The uranium ore mined today contains only 0.72$\%$ of fissionable ${^{235}}\mathrm{U}$ too little to make reactor fuel for thermal-neutron fission. For this reason, the mined ore must be enriched with $^{235} \mathrm{U}$
Both $$^{235} \mathrm{U}\left(T_{1 / 2}=7.0 \times 10^{8} \mathrm{y}\right)$$ and $$^{288} \mathrm{U}\left(T_{1 / 2}=4.5 \times 10^{9} \mathrm{y}\right)$$ are radioactive. How far back in time would natural uranium ore have been a practical reactor fuel, with a $^{235}\mathrm{U} / ^{238} \mathrm{U}$ ratio of 3.0$\% ?$

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02:59

Problem 30

Verify that the fusion of 1.0 $\mathrm{kg}$ of deuterium by the reaction $$
^{2} \mathrm{H}+^{2} \mathrm{H} \rightarrow^{3} \mathrm{Hc}+\mathrm{n} \quad(Q=+3.27 \mathrm{MeV})$$

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01:24

Problem 31

Calculate the height of the Coulomb barrier for the head-on collision of two deuterons, with effective radius $$2.1 \mathrm{fm} .$$

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03:04

Problem 32

For overcoming the Coulomb barrier for fusion, methods other than heating the fusible material have been suggested. For
example, if you were to use two particle accelerators to accelerate
two beams of deuterons directly toward each other so as to collide
head-on, (a) what voltage would each accelerator require in order for the colliding deuterons to overcome the Coulomb barrier? (b)Why do you suppose this method is not presently used?

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01:44

Problem 33

Calculate the Coulomb barrier height for two 7 Li nuclei that are fired at each other with the same initial kinetic energy $K .$ Hint: Use Eq. $42-3$ to calculate the radii of the nuclei.)

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02:15

Problem 34

In Fig. $43-10$ , the equation for $n(K),$ the number density per unit energy for particles, is
$$n(K)=1.13 n \frac{K^{1 / 2}}{(k T)^{3 / 2}} e^{-K / k T}$$ where $n$ is the total particle number density. At the center of the Sun, the temperature is $1.50 \times 10^{7} \mathrm{K}$ and the average proton en-
ergy $K_{\mathrm{ave}}$ is 1.94 $\mathrm{keV}$ . Find the ratio of the proton number density
at 5.00 $\mathrm{keV}$ to the number density at the average proton energy.

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01:42

Problem 35

Assume that the protons in a hot ball of protons each have a kinetic energy equal to $k T,$ where $k$ is the Boltzmann constant and
$T$ is the absolute temperature. If $T=1 \times 10^{7} \mathrm{K},$ what (approximately) is the least separation any two protons can have?

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01:20

Problem 36

What is the $Q$ of the following fusion process?
$$^{2} \mathrm{H}_{1}+^{1} \mathrm{H}_{1} \rightarrow^{3} \mathrm{He}_{2}+$$
Here are some atomic masses.
$$\begin{array}{llll}{^{2} \mathrm{H}_{1}} & {2.014102 \mathrm{u}} & {\mathrm{H}_{1}} & {1.007825 \mathrm{u}} \\ {^{3} \mathrm{He}_{2}} & {3.016029 \mathrm{u}}\end{array}$$

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02:23

Problem 37

The Sun has mass $2.0 \times 10^{30} \mathrm{kg}$ and radiates energy at the rate $3.9 \times 10^{26} \mathrm{W}$ (a) At what rate is its mass changing? (b) What
fraction of its original mass has it lost in this way since it began to
burn hydrogen, about $4.5 \times 10^{9} \mathrm{y}$ ago?

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01:32

Problem 38

We have seen that $Q$ for the overall proton-proton fusion cycle is 26.7 MeV. How can you relate this number to the $Q$ values for the reactions that make up this cycle, as displayed in Fig. $43-11 ?$

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00:53

Problem 39

Show that the energy released when three alpha particles fuse to form 12 $\mathrm{C}$ is 7.27 $\mathrm{MeV}$ . The atomic mass of 4 $\mathrm{He}$ is $4.0026 \mathrm{u},$
and that of $^{12} \mathrm{C}$ is 12.0000 $\mathrm{u} .$

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02:17

Problem 40

Calculate and compare the energy released by (a) the fusion of 1.0 $\mathrm{kg}$ of hydrogen deep within the Sun and (b) the fission of 1.0
$\mathrm{kg}$ of $^{25} \mathrm{U}$ in a fission reactor.

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02:31

Problem 41

A star converts all its hydrogen to helium, achieving a 100$\%$ helium composition. Next it converts the helium to carbon via the triple-alpha process,
$$^{4} \mathrm{He}+^{4} \mathrm{He}+^{4} \mathrm{He} \rightarrow^{12} \mathrm{C}+7.27 \mathrm{MeV}$$
The mass of the star is $4.6 \times 10^{32} \mathrm{kg}$ , and it generates energy at the
rate of $5.3 \times 10^{30} \mathrm{W}$ . How long will it take to convert all the helium
to carbon at this rate?

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01:50

Problem 42

Verify the three $Q$ values reported for the reactions given in Fig. $43-11 .$ The needed atomic and particle masses are $$^{1} \mathrm{H} \quad 1.007825 \mathrm{u}$$ $$
^{2} \mathrm{H} \quad 2.014102 \mathrm{u}$$ $$
^{3} \mathrm{He} \quad 3.016029 \mathrm{u}$$ $$
^{4} \mathrm{He} \quad 4.002603 \mathrm{u}$$ $$
\mathrm{e}^{ \pm} \quad 0.0005486 \mathrm{u}$$

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01:53

Problem 42

Verify the three $Q$ values reported for the reactions given in Fig. $43-11 .$ The needed atomic and particle masses are $$^{1} \mathrm{H} \quad 1.007825 \mathrm{u}$$ $$
^{2} \mathrm{H} \quad 2.014102 \mathrm{u}$$ $$
^{3} \mathrm{He} \quad 3.016029 \mathrm{u}$$ $$
^{4} \mathrm{He} \quad 4.002603 \mathrm{u}$$ $$
\mathrm{e}^{ \pm} \quad 0.0005486 \mathrm{u}$$
(Hint: Distinguish carefully between atomic and nuclear masses,
and take the positrons properly into account.)

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03:42

Problem 43

Figure $43-15$ shows an early proposal for a hydrogen bomb.The fusion fuel is deuterium, $^{2} \mathrm{H} .$ The high temperature and particle density needed for fusion are provided by an atomic bomb "trigger" that involves a 25 $\mathrm{U}$ or $^{239} \mathrm{Pu}$ fission fuel arranged to impress an imploding, compressive shock wave on the deuterium. The fusion reaction is
$$5^{2} \mathrm{H} \rightarrow^{3} \mathrm{He}+^{4} \mathrm{He}+^{1} \mathrm{H}+2 \mathrm{n}$$
(a) Calculate $Q$ for the fusion reaction. For needed atomic masses,
see Problem 42 . (b) Calculate the rating (see Problem 16) of the fu-
sion part of the bomb if it contains 500 $\mathrm{kg}$ of deuterium, 30.0$\%$ of
which undergoes fusion.

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01:33

Problem 44

Assume that the core of the Sun has one-eighth of the Sun's mass and is compressed within a sphere whose radius is one-fourth
of the solar radius. Assume further that the composition of the core
is 35$\%$ hydrogen by mass and that essentially all the Sun's energy
is generated there. If the Sun continues to burn hydrogen at the current rate of $6.2 \times 10^{11} \mathrm{kg} / \mathrm{s}$ , how long will it be before the hydrogen is entirely consumed? The Sun's mass is $2.0 \times 10^{30} \mathrm{kg}$ .

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02:41

Problem 45

(a) Calculate the rate at which the Sun generates neutrinos. Assume that energy production is entirely by the proton-proton fusion cycle. (b) At what rate do solar neutrinos reach Earth?

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01:45

Problem 46

In certain stars the carbon cvcle is more effective than the proton-proton cycle in generating energy. This carbon cycle is
$$^{12} \mathrm{C}+^{1} \mathrm{H} \rightarrow^{13} \mathrm{N}+\gamma, \quad Q_{1}=1.95 \mathrm{MeV}$$ $$
^{13} \mathrm{N} \rightarrow^{13} \mathrm{C}+\mathrm{e}^{+}+\nu, \quad Q_{2}=1.19$$$$
^{13} \mathrm{C}+^{1} \mathrm{H} \rightarrow^{14} \mathrm{N}+\gamma, \quad Q_{3}=7.55$$$$
^{14} \mathrm{N}+^{1} \mathrm{H} \rightarrow^{15} \mathrm{O}+\gamma, \quad Q_{4}=7.30$$$$
^{15} \mathrm{O} \rightarrow^{15} \mathrm{N}+\mathrm{e}^{+}+\nu, \quad Q_{5}=1.73$$$$
^{15} \mathrm{N}+^{1} \mathrm{H} \rightarrow^{12} \mathrm{C}+^{4} \mathrm{He}, \quad Q_{6}=4.97$$
(a) Show that this cycle is exactly equivalent in its overall effects to
the proton-proton cycle of Fig. $43-11$ . Verify that the two cycles, as expected, have the same $Q$ value.

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04:22

Problem 47

Coal burns according to the reaction $\mathrm{C}+\mathrm{O}_{2} \rightarrow$ $\mathrm{CO}_{2}$ . The heat of combustion is $3.3 \times 10^{7} \mathrm{J} / \mathrm{kg}$ of atomic carbon consumed. (a) Express this in terms of energy per carbon atom. (b)
Express it in terms of energy per kilogram of the initial reactants, carbon and oxygen. (c) Suppose that the Sun (mass $=2.0 \times 10^{30} \mathrm{kg}$ )
were made of carbon and oxygen in combustible proportions and
that it continued to radiate energy at its present rate of $3.9 \times 10^{26} \mathrm{W}$ .
How long would the Sun last?

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01:48

Problem 48

Verify the $Q$ values reported in Eqs. $43-13,43-14,$ and $43-15$ The needed masses are
$$1 \mathrm{H} \quad 1.007 \mathrm{s} 25 \mathrm{u}$$
$$^{2} \mathrm{H} \quad 2.014102 \mathrm{u}$$
$$^{3} \mathrm{H} \quad 3.016049 \mathrm{u}$$
$$4 \mathrm{He} \quad 4.002603 \mathrm{u}$$
$$\mathrm{n} \quad 1.008665 \mathrm{u}$$

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02:38

Problem 49

Roughly 0.0150$\%$ of the mass of ordinary water is due to "heavy water," in which one of the two hydrogens in an $\mathrm{H}_{2} \mathrm{O}$ molecule is replaced with deuterium, $^{2} \mathrm{H} .$ How much average fusion power could be obtained if we "burned" all the $^{2} \mathrm{H}$ in 1.00 liter of water in 1.00 day by somehow causing the deuterium to fuse via
the reaction $^{2} \mathrm{H}+^{2} \mathrm{H} \rightarrow^{3} \mathrm{He}+\mathrm{n}$ ?

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04:12

Problem 50

The effective $Q$ for the proton-proton cycle of Fig. $43-11$ is 26.2 MeV. (a) Express this as cnergy per kilogram of hydrogen consumed. (b) The power of the Sun is $3.9 \times 10^{26} \mathrm{W}$ . If its energy derives
from the proton-proton cycle, at what rate is it losing hydrogen? (c)
At what rate is it losing mass? (d) Account for the difference in the results for (b) and (c). (e) The mass of the Sun is $2.0 \times 10^{30}$ kg. If it loses mass at the constant rate calculated in (c), how long will it take to lose 0.10$\%$ of its mass?

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00:37

Problem 51

Many fear that nuclear power reactor technology will increase the likelihood of nuclear war because reactors can be used
not only to produce electrical energy but also, as a by-product through neutron capture with inexpensive $^{238} \mathrm{U},$ to make $^{299} \mathrm{Pu}$ which is a "fuel" for nuclear bombs. What simple series of reactions
involving neutron capture and beta decay would yield this plutonium isotope?

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02:07

Problem 52

In the deuteron-triton fusion reaction of Eq. $43-15,$ what is the kinetic energy of (a) the alpha particle and (b) the neutron? Neglect
the relatively small kinetic energies of the two combining particles.

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00:39

Problem 53

Verify that, as stated in Module $43-1,$ neutrons in equilibrium with matter at room temperature, 300 $\mathrm{K}$ , have an average kinetic energy of about 0.04 $\mathrm{eV} .$

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02:58

Problem 54

Verify that, as reported in Table $43-1,$ fissioning of the $^{255} \mathrm{U}$ in 1.0 $\mathrm{kg}$ of $$\mathrm{UO}_{2}$$ (enriched so that 235 $\mathrm{U}$ is 3.0$\%$ of the total uranium) could keep a 100 $\mathrm{W}$ lamp burning for 690 $\mathrm{y}$ .

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01:57

Problem 55

At the center of the Sun, the density of the gas is $1.5 \times 10^{5}$ $\mathrm{kg} / \mathrm{m}^{3}$ and the composition is essentially 35$\%$ hydrogen by mass
and 65$\%$ helium by mass. (a) What is the number density of protons there? (b) What is the ratio of that proton density to the density of particles in an ideal gas at standard temperature $\left(0^{\circ} \mathrm{C}\right)$ and pressure $\left(1.01 \times 10^{5} \mathrm{Pa}\right) ?$

Salamat Ali
Salamat Ali
Numerade Educator
04:08

Problem 56

Expressions for the Maxwell speed distribution for molecules in a gas are given in Chapter 19. (a) Show that the most probable energy is given by $$
K_{p}=\frac{1}{2} k T$$
Verify this result with the energy distribution curve of Fig. $43-10$ , for
which $T=1.5 \times 10^{7} \mathrm{K}$ (b) Show that the most probable speed is
given by
$$v_{p}=\sqrt{\frac{2 k T}{m}}$$
Find its value for protons at $T=1.5 \times 10^{7} \mathrm{K}$ . (c) Show that the
energy corresponding to the most probable speed (which is not the
same as the most probable energy is $$
K_{v, p}=k T$$
Locate this quantity on the curve of Fig. $43-10$

Ze-Han Lee
Ze-Han Lee
Numerade Educator
03:26

Problem 57

The uncompressed radius of the fuel pellet of Sample \begin{equation}
\begin{array}{l}{\text { Problem } 43.05 \text { is } 20 \mu \mathrm{m} \text { . Suppose that the compressed fuel pellet }} \\ {\text { "burns" with an efficiency of } 10 \%-\text { that is, only } 10 \% \text { of the }} \\ {\text { deuterons and } 10 \% \text { of the tritons participate in the fusion reaction }}\end{array}
\end{equation} of Eq. $43-15$ . (a) How much energy is released in each such mi-
croexplosion of a pellet? (b) To how much TNT is each such pellet
equivalent? The heat of combustion of TNT is 4.6 $\mathrm{MJ} / \mathrm{kg}$ . $\mathrm{c}$ ) If a fusion reactor is constructed on the basis of 100 microexplosions
per second, what power would be generated? (Part of this power
would be used to operate the lasers.)

Salamat Ali
Salamat Ali
Numerade Educator
01:24

Problem 58

Assume that a plasma temperature of $1 \times 10^{8} \mathrm{K}$ is reached in a laser-fusion device. (a) What is the most probable speed of a
deuteron at that temperature? (b) How far would such a deuteron
move in a confinement time of $1 \times 10^{-12} \mathrm{s} ?$

Ze-Han Lee
Ze-Han Lee
Numerade Educator