# Problems & Exercises

1

The energy of 30.0 $eVeV$ is required to ionize a molecule of the gas inside a Geiger tube, thereby producing an ion pair. Suppose a particle of ionizing radiation deposits 0.500 MeV of energy in this Geiger tube. What maximum number of ion pairs can it create?

2

A particle of ionizing radiation creates 4,000 ion pairs in the gas inside a Geiger tube as it passes through. What minimum energy was deposited, if 30.0 $eVeV$ is required to create each ion pair?

3

(a) Repeat Exercise 14.2, and convert the energy to joules or calories. (b) If all of this energy is converted to thermal energy in the gas, what is its temperature increase, assuming $50.0 cm350.0 cm3$ of ideal gas at 0.250-atm pressure? The small answer is consistent with the fact that the energy is large on a quantum mechanical scale but small on a macroscopic scale.

4

Suppose a particle of ionizing radiation deposits 1.0 MeV in the gas of a Geiger tube, all of which goes to creating ion pairs. Each ion pair requires 30.0 eV of energy. (a) The applied voltage sweeps the ions out of the gas in $1.00μs.1.00μs.$ What is the current? (b) This current is smaller than the actual current since the applied voltage in the Geiger tube accelerates the separated ions, which then create other ion pairs in subsequent collisions. What is the current if this last effect multiplies the number of ion pairs by 900?

### 14.2Substructure of the Nucleus

5

Verify that a $2.3×1017kg2.3×1017kg size 12{2 "." 3 times "10" rSup { size 8{"17"} } "kg"} {}$ mass of water at normal density would make a cube 60 km on a side, as claimed in Example 14.1. This mass at nuclear density would make a cube 1.0 m on a side.

6

Find the length of a side of a cube having a mass of 1.0 kg and the density of nuclear matter, taking this to be $2.3×1017 kg/m3.2.3×1017 kg/m3. size 12{2 "." 3´"10" rSup { size 8{"17"} } " kg/m" rSup { size 8{3} } } {}$

7

What is the radius of an $αα size 12{α} {}$particle?

8

Find the radius of a $238Pu238Pu size 12{"" lSup { size 8{"238"} } "Pu"} {}$ nucleus. $238Pu238Pu size 12{"" lSup { size 8{"238"} } "Pu"} {}$ is a manufactured nuclide that is used as a power source on some space probes.

9

(a) Calculate the radius of $58Ni,58Ni, size 12{"" lSup { size 8{"58"} } "Ni"} {}$ one of the most tightly bound stable nuclei.

(b) What is the ratio of the radius of $58Ni58Ni size 12{"" lSup { size 8{"58"} } "Ni"} {}$ to that of $258Ha,258Ha, size 12{"" lSup { size 8{"258"} } "Ha"} {}$ one of the largest nuclei ever made? Note that the radius of the largest nucleus is still much smaller than the size of an atom.

10

The unified atomic mass unit is defined to be $1 u=1.6605×10−27kg.1 u=1.6605×10−27kg. size 12{1" u"=1 "." "6605"×"10" rSup { size 8{-"27"} } "kg"} {}$ Verify that this amount of mass converted to energy yields 931.5 MeV. Note that you must use four-digit or better values for $cc size 12{c} {}$ and $∣qe∣.∣qe∣. size 12{ lline q rSub { size 8{e} } rline } {}$

11

What is the ratio of the velocity of a $ββ size 12{β} {}$particle to that of an $αα size 12{α} {}$particle, if they have the same nonrelativistic kinetic energy?

12

If a 1.50-cm-thick piece of lead can absorb 90.0 percent of the $γγ size 12{γ} {}$ rays from a radioactive source, how many centimeters of lead are needed to absorb all but 0.100 percent of the $γγ size 12{γ} {}$rays?

13

The detail observable using a probe is limited by its wavelength. Calculate the energy of a $γγ size 12{γ} {}$ray photon that has a wavelength of $1×10−16m,1×10−16m, size 12{1 times "10" rSup { size 8{ - "16"} } m} {}$ small enough to detect details about one-tenth the size of a nucleon. Note that a photon having this energy is difficult to produce and interacts poorly with the nucleus, limiting the practicability of this probe.

14

(a) Show that if you assume the average nucleus is spherical with a radius $r=r0A1/3,r=r0A1/3, size 12{r=r rSub { size 8{0} } A rSup { size 8{1/3} } } {}$ and with a mass of $AA size 12{A} {}$ u, then its density is independent of $A.A. size 12{A} {}$

(b) Calculate that density in $u/fm3u/fm3 size 12{"u/fm" rSup { size 8{3} } } {}$ and $kg/m3,kg/m3, size 12{"kg/m" rSup { size 8{3} } } {}$ and compare your results with those found in Example 14.1 for $56Fe.56Fe. size 12{"" lSup { size 8{"56"} } "Fe"} {}$

15

What is the ratio of the velocity of a 5.00-MeV $βrayβray size 12{β} {}$ to that of an $αα size 12{β} {}$particle with the same kinetic energy? This should confirm that βs travel much faster than $αα size 12{β} {}$s even when relativity is taken into consideration (see also Exercise 14.11).

16

(a) What is the kinetic energy in MeV of a $ββ size 12{β} {}$ray that is traveling at $0.998c?0.998c?$ This gives some idea of how energetic a $ββ size 12{β} {}$ray must be to travel at nearly the same speed as a $γγ$ray. (b) What is the velocity of the $γγ$ray relative to the $ββ size 12{β} {}$ray?

### 14.3Nuclear Decay and Conservation Laws

In the following eight problems, write the complete decay equation for the given nuclide in the complete $ZAXNZAXN size 12{"" lSub { size 8{Z} } lSup { size 8{A} } X rSub { size 8{N} } } {}$ notation. Refer to the periodic table for values of $Z.Z. size 12{Z} {}$

17

$β−β− size 12{β rSup { size 8{ - {}} } } {}$ decay of $3H3H size 12{"" lSup { size 8{3} } H} {}$ (tritium), a manufactured isotope of hydrogen used in some digital watch displays, and manufactured primarily for use in hydrogen bombs

18

$β−β− size 12{β rSup { size 8{ - {}} } } {}$ decay of $40K,40K, size 12{"" lSup { size 8{"40"} } K} {}$ a naturally occurring rare isotope of potassium responsible for some of our exposure to background radiation

19

$β+β+ size 12{β rSup { size 8{+{}} } } {}$ decay of $50Mn50Mn size 12{"" lSup { size 8{"50"} } "Mn"} {}$

20

$β+β+ size 12{β rSup { size 8{+{}} } } {}$ decay of $52Fe52Fe size 12{"" lSup { size 8{"52"} } "Fe"} {}$

21

electron capture by $7Be7Be size 12{"" lSup { size 8{7} } "Be"} {}$

22

electron capture by $106In106In size 12{"" lSup { size 8{"106"} } "In"} {}$

23

$αα size 12{α} {}$ decay of $210Po,210Po, size 12{"" lSup { size 8{"210"} } "Po"} {}$ the isotope of polonium in the decay series of $238U238U size 12{"" lSup { size 8{"238"} } U} {}$ that was discovered by the Curies; a favorite isotope in physics labs, since it has a short half-life and decays to a stable nuclide

24

$αα size 12{α} {}$ decay of $226Ra,226Ra, size 12{"" lSup { size 8{"226"} } "Ra"} {}$ another isotope in the decay series of $238U,238U, size 12{"" lSup { size 8{"238"} } U} {}$ first recognized as a new element by the Curies; poses special problems because its daughter is a radioactive noble gas

In the following four problems, identify the parent nuclide and write the complete decay equation in the $ZAXNZAXN size 12{"" lSub { size 8{Z} } lSup { size 8{A} } X rSub { size 8{N} } } {}$ notation. Refer to the periodic table for values of $Z.Z. size 12{Z} {}$

25

$β−β− size 12{β rSup { size 8{ - {}} } } {}$ decay producing $137Ba;137Ba; size 12{"" lSup { size 8{"137"} } "Ba"} {}$ The parent nuclide is a major waste product of reactors and has chemistry similar to potassium and sodium, resulting in its concentration in your cells if ingested.

26

$β−β− size 12{β rSup { size 8{ - {}} } } {}$ decay producing $90Y;90Y; size 12{"" lSup { size 8{"90"} } Y} {}$ The parent nuclide is a major waste product of reactors and has chemistry similar to calcium, so that it is concentrated in bones if ingested$—90Y—90Y size 12{"" lSup { size 8{"90"} } Y} {}$ is also radioactive.

27

$αα size 12{α} {}$ decay producing $228Ra;228Ra; size 12{"" lSup { size 8{"228"} } "Ra"} {}$ The parent nuclide is nearly 100 percent of the natural element and is found in gas lantern mantles and in metal alloys used in jets—$228Ra228Ra size 12{"" lSup { size 8{"228"} } "Ra"} {}$ is also radioactive.

28

$αα size 12{α} {}$ decay producing $208Pb;208Pb; size 12{"" lSup { size 8{"208"} } "Pb"} {}$ The parent nuclide is in the decay series produced by $232Th,232Th, size 12{"" lSup { size 8{"232"} } "Th"} {}$ the only naturally occurring isotope of thorium.

29

When an electron and positron annihilate, both their masses are destroyed, creating two equal energy photons to preserve momentum. (a) Confirm that the annihilation equation $e++e−→γ+γe++e−→γ+γ size 12{e rSup { size 8{+{}} } +e rSup { size 8{ - {}} } rightarrow γ+γ} {}$ conserves charge, electron family number, and total number of nucleons. To do this, identify the values of each before and after the annihilation. (b) Find the energy of each $γγ size 12{γ} {}$ ray, assuming the electron and positron are initially nearly at rest. (c) Explain why the two $γγ size 12{γ} {}$ rays travel in exactly opposite directions if the center of mass of the electron-positron system is initially at rest.

30

Confirm that charge, electron family number, and the total number of nucleons are all conserved by the rule for $αα$ decay given in the equation $ZAXN→ Z−2A−4 YN−2 + 24 He2 .ZAXN→ Z−2A−4 YN−2 + 24 He2 .$ To do this, identify the values of each before and after the decay.

31

Confirm that charge, electron family number, and the total number of nucleons are all conserved by the rule for $β−β−$ decay given in the equation $ZA XN → Z+1A Y N−1 + β− + ν¯e.ZA XN → Z+1A Y N−1 + β− + ν¯e. size 12{"" lSub { size 8{Z} } lSup { size 8{A} } X rSub { size 8{N} } rightarrow "" lSub { size 8{Z−1} } lSup { size 8{A} } Y rSub { size 8{N - 1} } +β rSup { size 8{ - {}} } + {overline {v rSub { size 8{e} } }} } {}$ To do this, identify the values of each before and after the decay.

32

Confirm that charge, electron family number, and the total number of nucleons are all conserved by the rule for $β−β− size 12{β rSup { size 8{ - {}} } } {}$ decay given in the equation $ZA XN → Z−1A Y N−1 + β− + νe.ZA XN → Z−1A Y N−1 + β− + νe.$ To do this, identify the values of each before and after the decay.

33

Confirm that charge, electron family number, and the total number of nucleons are all conserved by the rule for electron capture given in the equation $ZAXN+e−→Z−1AYN+1+νe.ZAXN+e−→Z−1AYN+1+νe. size 12{"" lSub { size 8{Z} } lSup { size 8{A} } X rSub { size 8{N} } +e rSup { size 8{ - {}} } rightarrow "" lSub { size 8{Z - 1} } lSup { size 8{A} } Y rSub { size 8{N+1} } +v rSub { size 8{e} } } {}$ To do this, identify the values of each before and after the capture.

34

A rare decay mode has been observed in which $222Ra222Ra$ emits a $14C14C$ nucleus. (a) The decay equation is $222Ra→AX+14C.222Ra→AX+14C. size 12{ {} rSup { size 8{"222"} } "Ra" rightarrow rSup { size 8{A} } "X+" rSup { size 8{"14"} } C} {}$ Identify the nuclide $AX.AX.$ (b) Find the energy emitted in the decay. The mass of $222Ra222Ra size 12{"" lSup { size 8{"222"} } "Ra"} {}$ is 222.015353 u.

35

(a) Write the complete $αα size 12{α} {}$ decay equation for $226Ra.226Ra. size 12{"" lSup { size 8{"226"} } "Ra"} {}$

(b) Find the energy released in the decay.

36

(a) Write the complete $αα size 12{α} {}$ decay equation for $249Cf.249Cf. size 12{"" lSup { size 8{"249"} } "Cf"} {}$

(b) Find the energy released in the decay.

37

(a) Write the complete $β−β− size 12{β rSup { size 8{ - {}} } } {}$ decay equation for the neutron. (b) Find the energy released in the decay.

38

(a) Write the complete $β−β− size 12{β rSup { size 8{ - {}} } } {}$ decay equation for $90Sr,90Sr, size 12{"" lSup { size 8{"90"} } "Sr"} {}$ a major waste product of nuclear reactors. (b) Find the energy released in the decay.

39

Calculate the energy released in the $β+β+ size 12{β rSup { size 8{+{}} } } {}$ decay of $22Na,22Na,$ the equation for which is given in the text. The masses of $22Na22Na$ and $22Ne22Ne size 12{"" lSup { size 8{"22"} } "Ne"} {}$ are 21.994434 and 21.991383 u, respectively.

40

(a) Write the complete $β+β+ size 12{β rSup { size 8{+{}} } } {}$ decay equation for $11C.11C. size 12{"" lSup { size 8{"11"} } C} {}$

(b) Calculate the energy released in the decay. The masses of $11C11C size 12{"" lSup { size 8{"11"} } C} {}$ and $11B11B size 12{"" lSup { size 8{"11"} } B} {}$ are 11.011433 and 11.009305 u, respectively.

41

(a) Calculate the energy released in the $αα size 12{α} {}$ decay of $238U.238U. size 12{"" lSup { size 8{"238"} } U} {}$

(b) What fraction of the mass of a single $238U238U size 12{"" lSup { size 8{"238"} } U} {}$ is destroyed in the decay? The mass of $234Th234Th size 12{"" lSup { size 8{"234"} } "Th"} {}$ is 234.043593 u.

(c) Although the fractional mass loss is large for a single nucleus, it is difficult to observe for an entire macroscopic sample of uranium. Why is this?

42

(a) Write the complete reaction equation for electron capture by $7Be.7Be. size 12{"" lSup { size 8{7} } "Be"} {}$

(b) Calculate the energy released.

43

(a) Write the complete reaction equation for electron capture by $15O.15O. size 12{"" lSup { size 8{"15"} } O} {}$

(b) Calculate the energy released.

### 14.4Half-Life and Activity

Data from the appendices and the periodic table may be needed for these problems.

44

An old campfire is uncovered during an archaeological dig. Its charcoal is found to contain less than 1/1,000 the normal amount of $14C.14C. size 12{"" lSup { size 8{"14"} } C} {}$ Estimate the minimum age of the charcoal, noting that $210=1,024.210=1,024. size 12{2 rSup { size 8{"10"} } ="1024"} {}$

45

A $60Co60Co size 12{"" lSup { size 8{"60"} } "Co"} {}$ source is labeled 4.00 mCi, but its present activity is found to be $1.85×1071.85×107 size 12{1 "." "85" times "10" rSup { size 8{7} } } {}$ Bq. (a) What is the present activity in mCi? (b) How long ago did it actually have a 4.00-mCi activity?

46

(a) Calculate the activity $RR size 12{R} {}$ in curies of 1.00 g of $226Ra.226Ra. size 12{"" lSup { size 8{"226"} } "Ra"} {}$ (b) Discuss why your answer is not exactly 1.00 Ci, given that the curie was originally supposed to be exactly the activity of a gram of radium.

47

Show that the activity of the $14C14C size 12{"" lSup { size 8{"14"} } C} {}$ in 1.00 g of $12C12C size 12{"" lSup { size 8{"12"} } C} {}$ found in living tissue is 0.250 Bq.

48

Mantles for gas lanterns contain thorium, because it forms an oxide that can survive being heated to incandescence for long periods of time. Natural thorium is almost 100 percent $232Th,232Th, size 12{"" lSup { size 8{"232"} } "Th"} {}$ with a half-life of $1.405×1010y.1.405×1010y. size 12{1 "." "405" times "10" rSup { size 8{"10"} } } {}$ If an average lantern mantle contains 300 mg of thorium, what is its activity?

49

Cow’s milk produced near nuclear reactors can be tested for as little as 1.00 pCi of $131I131I size 12{"" lSup { size 8{"131"} } I} {}$ per liter, to check for possible reactor leakage. What mass of $131I131I size 12{"" lSup { size 8{"131"} } I} {}$ has this activity?

50

(a) Natural potassium contains $40K,40K,$ which has a half-life of $1.277×1091.277×109 size 12{1 "." "277" times "10" rSup { size 8{9} } } {}$ y. What mass of $40K40K size 12{"" lSup { size 8{"40"} } K} {}$ in a person would have a decay rate of 4,140 Bq? (b) What is the fraction of $40K40K size 12{"" lSup { size 8{"40"} } K} {}$ in natural potassium, given that the person has 140 g in his body? These numbers are typical for a 70-kg adult.

51

There is more than one isotope of natural uranium. If a researcher isolates 1.00 mg of the relatively scarce $235U235U size 12{"" lSup { size 8{"235"} } U} {}$ and finds this mass to have an activity of 80.0 Bq, what is its half-life in years?

52

$50V50V$ has one of the longest known radioactive half-lives. In a difficult experiment, a researcher found that the activity of 1.00 kg of $50V50V$ is 1.75 Bq. What is the half-life in years?

53

You can sometimes find deep red crystal vases in antique stores, called uranium glass because their color was produced by doping the glass with uranium. Look up the natural isotopes of uranium and their half-lives, and calculate the activity of such a vase assuming it has 2.00 g of uranium in it. Neglect the activity of any daughter nuclides.

54

A tree falls in a forest. How many years must pass before the $14C14C size 12{"" lSup { size 8{"14"} } C} {}$ activity in 1.00 g of the tree’s carbon drops to 1.00 decay per hour?

55

What fraction of the $40K40K size 12{"" lSup { size 8{"40"} } K} {}$ that was on Earth when it formed $4.5×1094.5×109$ years ago is left today?

56

A 5,000-Ci $60Co60Co$ source used for medical therapy is considered too weak to be useful when its activity falls to 3,500 Ci. How long after its manufacture does this happen?

57

Natural uranium is 0.7200 percent $235U235U$ and 99.27 percent $238U.238U.$ What were the percentages of $235U235U$ and $238U238U$ in natural uranium when Earth formed $4.5×1094.5×109$ years ago?

58

The $β−particlesβ−particles size 12{β rSup { size 8{ - {}} } } {}$ emitted in the decay of $3H3H$ (tritium) interact with matter to create light in a glow-in-the-dark exit sign. At the time of manufacture, such a sign contains 15.0 Ci of $3H.3H. size 12{"" lSup { size 8{3} } H} {}$ (a) What is the mass of the tritium? (b) What is its activity 5.00 y after manufacture?

59

World War II aircraft had instruments with glowing radium-painted dials. The activity of one such instrument was $1.0×1051.0×105$ Bq when new. (a) What mass of $226Ra226Ra$ was present? (b) After some years, the phosphors on the dials deteriorated chemically, but the radium did not escape. What is the activity of this instrument 57.0 years after it was made?

60

(a) The $210Po210Po$ source used in a physics laboratory is labeled as having an activity of $1.0μCi1.0μCi size 12{1 "." 0 m"Ci"} {}$ on the date it was prepared. A student measures the radioactivity of this source with a Geiger counter and observes 1,500 counts per minute. She notices that the source was prepared 120 days before her lab. What fraction of the decays is she observing with her apparatus? (b) Identify some of the reasons that only a fraction of the αs emitted are observed by the detector.

61

Armor-piercing shells with depleted uranium cores are fired by aircraft at tanks. The high density of the uranium makes them effective. The uranium is called depleted because it has had its $235U235U$ removed for reactor use and is nearly pure $238U.238U.$ Depleted uranium has been erroneously called nonradioactive. To demonstrate that this is wrong: (a) Calculate the activity of 60.0 g of pure $238U.238U. size 12{"" lSup { size 8{"238"} } U} {}$ (b) Calculate the activity of 60.0 g of natural uranium, neglecting the $234U234U$ and all daughter nuclides.

62

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

63

Large amounts of depleted uranium $( 238 U) ( 238 U)$ 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 4,000-kg block of $238 U. 238 U.$ (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.

64

The Galileo space probe was launched on its long journey past several planets in 1989, with an ultimate goal of Jupiter. Its power source is 11.0 kg of $238 Pu, 238 Pu,$ a by-product of nuclear weapons plutonium production. Electrical energy is generated thermoelectrically from the heat produced when the 5.59-MeV $α α$particles emitted in each decay crash to a halt inside the plutonium and its shielding. The half-life of $238 Pu 238 Pu$ is 87.7 years. (a) What was the original activity of the $238 Pu 238 Pu$ in becquerel? (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 $γ γ$rays.

65

Consider the generation of electricity by a radioactive isotope in a space probe, such as described in Exercise 14.64. Construct a problem in which you calculate the mass of a radioactive isotope you need in order to supply power for a long space flight. Among the things to consider are the isotope chosen, its half-life and decay energy, the power needs of the probe and the length of the flight.

66

Unreasonable Results

A nuclear physicist finds $1.0 μg 1.0μg$ of $236 U 236 U$ in a piece of uranium ore and assumes it is primordial since its half-life is $2.3 × 10 7 y. 2.3× 10 7 y.$ (a) Calculate the amount of $236 U 236 U$that would had to have been on Earth when it formed $4.5 × 10 9 y 4.5× 10 9 y$ ago for $1.0 μg 1.0μg$ to be left today. (b) What is unreasonable about this result? (c) What assumption is responsible?

67

Unreasonable Results

(a) Repeat Exercise 14.57 but include the 0.0055 percent natural abundance of $234 U 234 U$ with its $2.45 × 10 5 y 2.45× 10 5 y$ half-life. (b) What is unreasonable about this result? (c) What assumption is responsible? (d) Where does the $234 U 234 U$ come from if it is not primordial?

68

Unreasonable Results

The manufacturer of a smoke alarm decides that the smallest current of $α α$ radiation he can detect is $1.00 μA. 1.00μA.$ (a) Find the activity in curies of an $α α$ emitter that produces a $1.00 μA 1.00μA$ current of $α α$particles. (b) What is unreasonable about this result? (c) What assumption is responsible?

69

Different unstable elements have vastly diverse half-lives. These differences in half-lives make different unstable elements and isotopes appropriate for use in a variety of applications. How is this difference in half-lives used? Under what circumstances is it advantageous to utilize an element with a short half-life? A long one?

### 14.5Binding Energy

70

$2H2H$ is a loosely bound isotope of hydrogen. Called deuterium or heavy hydrogen, it is stable but relatively rare—it is 0.015 percent of natural hydrogen. Note that deuterium has $Z=N,Z=N, size 12{Z=N} {}$ which should tend to make it more tightly bound, but both are odd numbers. Calculate $BE/A,BE/A,$ the binding energy per nucleon, for $2H2H$ and compare it with the approximate value obtained from the graph in Figure 14.21.

71

$56Fe56Fe$ is among the most tightly bound of all nuclides. It is more than 90 percent of natural iron. Note that $56Fe56Fe$ has even numbers of both protons and neutrons. Calculate $BE/A,BE/A,$ the binding energy per nucleon, for $56Fe56Fe$ and compare it with the approximate value obtained from the graph in Figure 14.21.

72

$209Bi209Bi$ is the heaviest stable nuclide, and its $BE/ABE/A$ is low compared with medium-mass nuclides. Calculate $BE/A,BE/A,$ the binding energy per nucleon, for $209Bi209Bi$ and compare it with the approximate value obtained from the graph in Figure 14.21.

73

(a) Calculate $BE/ABE/A$ for $235U,235U,$ the rarer of the two most common uranium isotopes. (b) Calculate $BE/ABE/A$ for $238U.238U.$ Most of uranium is $238U.238U.$ Note that $238U238U$ has even numbers of both protons and neutrons. Is the $BE/ABE/A$ of $238U238U$ significantly different from that of $235U?235U?$

74

(a) Calculate $BE/ABE/A$ for $12C.12C.$ Stable and relatively tightly bound, this nuclide is most of natural carbon. (b) Calculate $BE/ABE/A$ for $14C.14C.$ Is the difference in $BE/ABE/A$ between $12C12C$ and $14C14C$ significant? One is stable and common, and the other is unstable and rare.

75

The fact that $BE/ABE/A$ is greatest for $AA$ near 60 implies that the range of the nuclear force is about the diameter of such nuclides. (a) Calculate the diameter of an $A=60A=60$ nucleus. (b) Compare $BE/ABE/A$ for $58Ni58Ni$ and $90Sr.90Sr.$ The first is one of the most tightly bound nuclides, while the second is larger and less tightly bound.

76

The purpose of this problem is to show in three ways that the binding energy of the electron in a hydrogen atom is negligible compared with the masses 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 mass of the hydrogen atom obtained from Exercise 17.31. (b) Subtract the mass of the proton given in Table 14.1 from the mass of the hydrogen atom given in Exercise 17.31. You will find the difference is equal to the electron’s mass to three digits, implying the binding energy is small in comparison. (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.

77

Unreasonable Results

A particle physicist discovers a neutral particle with a mass of 2.02733 u that he assumes is two neutrons bound together. (a) Find the binding energy. (b) What is unreasonable about this result? (c) What assumptions are unreasonable or inconsistent?

### 14.6Tunneling

78

Derive an approximate relationship between the energy of $αα$ decay and half-life using the following data. It may be useful to graph the log of $t1/2t1/2$ against $EαEα$ to find some straight-line relationship.

Nuclide $E α (MeV) E α (MeV)$ $t 1/2 t 1/2$
$216 Ra 216 Ra$ $9.5 9.5$ $0.18 μs 0.18 μs$
$194 Po 194 Po$ $7.0 7.0$ $0.7 s 0.7 s$
$240 Cm 240 Cm$ $6.4 6.4$ $27 d 27 d$
$226 Ra 226 Ra$ $4.91 4.91$ $1600 y 1600 y$
$232 Th 232 Th$ $4.1 4.1$ $1.4 × 10 10 y 1.4 × 10 10 y$
Table 14.2 Energy and Half-Life for $α α size 12{α} {}$ Decay
79

Integrated Concepts

A 2.00-T magnetic field is applied perpendicular to the path of charged particles in a bubble chamber. What is the radius of curvature of the path of a 10 MeV proton in this field? Neglect any slowing along its path.

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(a) Write the decay equation for the $αα$ decay of $235 U. 235 U.$ (b) What energy is released in this decay? The mass of the daughter nuclide is 231.036298 u. (c) Assuming the residual nucleus is formed in its ground state, how much energy goes to the $αα$particle?

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Unreasonable Results

The relatively scarce naturally occurring calcium isotope $48 Ca 48 Ca$ has a half-life of about $2×1016y.2×1016y.$ (a) A small sample of this isotope is labeled as having an activity of 1.0 Ci. What is the mass of the $48 Ca 48 Ca$ in the sample? (b) What is unreasonable about this result? (c) What assumption is responsible?

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Unreasonable Results

A physicist scatters $γγ$ rays from a substance and sees evidence of a nucleus $7.5×10–13m7.5×10–13m$ in radius. (a) Find the atomic mass of such a nucleus. (b) What is unreasonable about this result? (c) What is unreasonable about the assumption?

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Unreasonable Results

A frazzled theoretical physicist reckons that all conservation laws are obeyed in the decay of a proton into a neutron, positron, and neutrino—as in $β+β+$ decay of a nucleus—and sends a paper to a journal to announce the reaction as a possible end of the universe due to the spontaneous decay of protons. (a) What energy is released in this decay? (b) What is unreasonable about this result? (c) What assumption is responsible?

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