Chapter 1 Solution 24 2.14) The planet earth has a mass of approximately 6 ×10 kg . If the density of the earth were e
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Chapter 1 Solution 24 2.14) The planet earth has a mass of approximately 6 ×10 kg . If the density of the earth were equal to that of nuclei, how big would the earth be?
Solution: For one nucleus density;
R = 1.25 × 10 −13 cm × A1/ 3 = 1.25 × 10 −13 cm Assuming that the nuclei have spherical shape;
V=
4π 3 R = 8.181× 10−39 cm3 3
ma.m.u = 1.66054 × 10 −24 g ⇒ ρ = 2.03 × 1017 kg / m3 Thus, the earth volume becomes;
Vearth =
mearth
ρ
=
6 ×1024 kg = 2.96 ×107 m3 2.03 ×1017 kg / m3 ( V
real
= 1.0832 × 10 21 m3 )
If the density of the Earth were equal to the density of the nuclei, its volume would be as small as
2.733 × 10 −16 of its present volume. 2.16) The fission of the nucleus of
235
U releases approximately 200MeV. How much energy (in kilowatt‐
hours and megawatt‐days) is released when 1 g of
235
U undergoes fission?
Solution:
1g − 235 U =
1g × 0.6022 × 1024 fissioned − 235 Uatoms = 2.563 ×1021 atoms 235 g 2.563 × 10 21 atoms ×
Released Energy:
200 MeV = 512.6 × 1021 MeV 1atoms
4.450 × 10−20 kWhr = 512.6 ×10 MeV × = 22810kWhr 1MeV 21
= 22810kWhr ×
1day = 0.9505MWd 24hr
2.25) An electron moves with a kinetic energy equal to its rest‐mass energy. Calculate the electron’s a) total energy in units of me c 2 b) mass in units of
me
c) speed in units of c d) wavelength in units of the Compton wavelength Solution:
Erest = me c 2
and
Erest = Ekinetic ⇒ Etot = Ekinetic + Erest = 2me c 2
divided − by − c Etot = Ekinetic + Erest ⇒ mtot c 2 = me c 2 + me c 2 ⎯⎯⎯⎯⎯ → mtot = 2me 2
m me 1 ⇒ ν = (1 − ( e ) 2 )1/ 2 c = 0.86603c m= = 2 2 m 1 −ν / c where m 2 λ hc hc h λ= = = = c = 0.5774λc 3me c 3 4me 2 c 4 − me 2 c 4 Etotal 2 − Erest 2
me
where λc =
h me c
2.39) Consider the chain decay
A → B → C , With no atoms of B present at t=0. a) Show that the activity of B rises to a maximum value at the time tm given by
tm =
1 λ ln( B ) λB − λ A λ A
At which time the activities of A and B are equal. b) Show that, for t < tm , activity of B is less than that of A, whereas the reverse is the case for t > tm . Solution:
NB = a)
N A (0)λA − λAt − λB t (e − e ) λ B − λA
dN B =0 The maximum value at dt
− λ A e − λ A t + λ B e − λB t = 0 ⇒ λ A e − λ A t = λ B e − λ B t ln(λA e − λAt ) = ln(λ B e − λBt ) ⇒ ln(λ A ) − λ At = ln(λB ) − λB t ln(λ B e − λB t )
− ln(λ A ) + ln(λB ) = λB t − λAt
⇒t =
1 λ ln( B ) λB − λ A λ A
Test if activities equal
N A λ A = N B λB N A (0)λ A e − λAt =
N A (0)λB λ A − λAt − λB t λA λA ln( λB / λ A ) ⇒ (e − e ) = ( λB − λA ) λB − λ A λ λB λB − λ A B ⇒ λA = λB e
b) From the above equation if RHS is greater, then activity, B is greater if LHS is greater, then activity A is greater. So, consider λA
λA
B
B
− ε ln( ) − ln( λA =e λ =e λ λB
)ε
=(
λA ε ) note ε tm is substitute instead of tm . λB
So if ε >1, t > tm and activity of B is greater, if ε 1 1), the concen ntration of the i‐th radionu uclide can be determined from
λ1 (U − T1/1 2 = 244.5kyy ) = 7.77 × 100−9 d −1 λ2 (Th − T1/ 2 = 77 ky ) = 24.66 × 10−9 d −1 λ3 (Ra − T1/ 2 = 1.6ky ) = 1.19 × 10−6 d −1
λ4 (Rn − T1/ 2 = 3.8d ) = 0.1824d −1 N Rn (0) =
λ1
λ2
λ3
λ2 − λ1 λ3 − λ2 λ4 − λ3
NU (0) = 8.7664 × 1015 atom ms
ARn (0) = λ4 × N Rn (0) = 1.866 × 10110 Bq 2.51) Consider the reaction 6
Li (α , p )9 Be
Using atomic mass datta, compute: 6 9 4 H a) the total binding energy of Li , Be and He
b) the Q vvalue of the re eaction usingg the results o of part (a). Solution:
6
Decay Scheme: 3 a)
Li + 42 He →94 Be +11 p
BE ( a ) = Z a M (1 H ) + N a M n − M a
a is in units of mass for a + b → c + d
BE (36 Li) = Z Li M (1 H ) + N Li M n − M Li = (3 ×1.00797 + 3 ×1.008665 − 6.01512)amu × 931.494MeV / amu = 32.40MeV BE ( 42 He) = Z He M (1 H ) + N He M n − M He = (3 × 1.00797 + 3 × 1.008665 − 4.0026) amu × 931.494 MeV / amu = 29 MeV BE (94 Be) = Z Be M (1 H ) + N Be M n − M Be = (3 × 1.00797 + 3 × 1.008665 − 9.0122) amu × 931.494 MeV / amu = 59 MeV
b) Q = [ BE (c ) + BE ( d )] − [ BE ( a ) + BE (b)] Or Q = [ M a + M b ] − [ M c + M d ]931MeV
⇒ Q = [ BE ( Be)] − [ BE ( Li ) + BE (α )] = 59 − (32.4 + 29) = −2.4 MeV 2.54) Compute the separation energies of the last neutron in the following nuclei: 4 a) He 7 b) Li 17 c) O 51 d) V
e) f)
208
235
Pb
U
Solution: The separation energy of the last neutron can be estimated from;
ES = [ M n + M ( A−1 Z ) − M ( A Z )] × 931.481MeV / amu 4
a) 2
He : 42 He →10 n + 32 He
7
b) 3 17
c) 8
Li : 37 Li →10 n + 36 Li
and
and
ES = −[4.0026 − 1.008665 − 3.01603] × 931.48 = 20.581MeV
ES = −[7.016 − 1.008665 − 6.01512] × 931.48 = 7.2516 MeV
1 16 O : 17 8 O → 0 n + 8 O and ES = −[16.9913 − 1.008665 − 15.99491] × 931.48 = 4.150 MeV
51
d) 23
1 50 V : 51 23 V → 0 n + 23 V
208
e) 82 235
f) 92
and
208 207 Pb : 82 Pb →10 n + 82 Pb
ES = −[50.9440 − 1.008665 − 49.9472] × 931.48 = 11.052 MeV
and
ES = −[207.9766 − 1.008665 − 206.9759] × 931.48 = 7.419 MeV
235 234 U : 92 U →10 n + 92 U and ES = −[235.0439 − 1.008665 − 234.0409] × 931.48 = 5.2768MeV
2.61) It has been proposed to use uranium carbide (UC) for the initial fuel in certain types of breeder 3
reactors, with the uranium enriched to 25 w/o. The density of UC is 13.6 g / cm . a) What is the atomic weight of the uranium? b) What is the atomic density of the
235
U ?
Solution:
ωi 1 1 1 1 25 75 ( ) ⇒ M = 237.292g = ⇒ = + ∑ 100 100 235.0439 238.0508 M M i Mi a) b) M (UC ) = M (U ) + M (C ) = 237.292 + 12 = 249.292 g The percent by weight of uranium in UC = 237.292 ×100/249.292=95.186w/o Average density of the uranium is therefore =0.95186 × 13.6=12.945g/cm 3
And the density of U‐235 =0.25 × 12.945=3.236g/cm 3
ρ NA
3.236 × 0.6022 × 1024 N( U) = = = 8.2917 × 1021 atoms / cm3 M 235.0439 235
2.62) Compute the atom densities of uranium is enriched to 3.5w / o in
235
235
3
U and 238U in UO of physical density 10.8 g / cm if the 2
U .
Solution: It is first necessary to compute the atomic weight of the uranium.
1 1 3.5 96.5 = + ( ) ⇒ M = 237.944 M 100 235.0439 238.0508 The molecular weight of the UO2 is then; MUO2 = 237.944+2 × 15.999=269.942
The percent by weight of uranium in UO2 = 237.944 ×100/269.942=88.146w/o Average density of the uranium is therefore =0.88146 × 10.8=9.5198g/cm 3
And the density of U‐235 =0.035 × 9.5198=0.333193g/cm 3
N ( 235U ) =
ρ NA M
=
0.33193 × 0.6022 ×1024 = 8.504 × 1020 atoms / cm3 235.0439
And the density of U‐238 =0.965 × 9.5198=9.1866g/cm 3
N ( 238U ) =
ρ NA M
=
9.1866 × 0.6022 × 1024 = 2.323 × 1022 atoms / cm3 238.0504
2.63) The fuel for certain breeder reactor consist of pellets composed of mixed oxides, UO2 and PuO2 , with the PuO2 compromising approximately 30 w/o of the mixture. The uranium essentially all
238
U ,
whereas the plutonium contains the following isotopes: 239 Pu (70.5w / o) , 240 Pu (21.3w / o) , 241
Pu (5.5w / o) and 242 Pu (2.7 w / o) . Calculate the number of atoms each isotope per gram of the
fuel. Solution:
ωi 1 1 1 1 70.5 21.3 5.5 2.7 = ⇒ = + + + ( ) ⇒ M Pu = 239.4 ∑ M 100 i M i M Pu 100 239 240 241 242 M PuO2 = 271.4
1 M (UPu )O2
=
and
M UO2 = 270
1 30 70 ( + ) ⇒ M (UPu )O2 = 270.4 100 271.4 270
There are 0.3 g
PuO2 and 0.7g UO2 in 1g of fuel. Thus,
The percent by weight of Pu in PuO2 = 239.4 ×100/271.4=88.21w/o The percent by weight of Pu‐239 in PuO2 = 0.705 × 88.21 = 62.19 w / o The percent by weight of Pu‐240 in PuO2 = 0.213 × 88.21 = 18.79w / o The percent by weight of Pu‐241 in PuO2 = 4.85w / o
The percent by weight of Pu‐241 in PuO2 = 2.38w / o The percent by weight of U‐238 in UO2 = 238 ×100/270=88.15w/o So,
N Pu − 239 =
0.3 × 0.6219 × 0.6022 × 1024 = 4.7 ×1020 atoms 239
N Pu − 240 =
0.3 × 0.1879 × 0.6022 × 1024 = 1.41× 1020 atoms 239
0.3 × 0.0485 × 0.6022 × 1024 = 3.64 ×1019 atoms N Pu − 241 = 239 N Pu − 242 =
0.3 × 0.0238 × 0.6022 ×1024 = 1.78 ×1019 atoms 239
NU − 238 =
0.7 × 0.8815 × 0.6022 ×1024 = 1.56 × 1021 atoms 239
Chapter 3 Solution 3.1) Two beams of 1‐eV neutrons intersect at an angle of 90. The density of neutrons is both beams is
2 × 108 neutrons / cm 3 . a) Calculate the intensity of each beam. b) What is the neutron flux where the two beams intersects? Solution:
n = 2 × 108 neutrons / cm 3
ν=
2E m where E = 1 − eV and m = 939.56563MeV / c 2 for neutron
⇒ ν = 13831.6m / s I = nν = 2.7663 × 1014 neutrons / cm 2 − sec
φ = ntotν = ( n1 + n2 )ν = 5.5365 × 1014 neutrons / cm 2 − sec
10 2 2 3.3) A monoenergetic neutron beams , φ = 4 × 10 n / cm − sec , impinges on a target 1cm in area 24 3 and 0.1cm thick. There are 0.048 × 10 atoms / cm in the target, and the total cross‐section at the energy of the beam is 4.5b.
(a) What is the macroscopic total cross‐section? (b) How many neutron interactions per second occur in the target? (c) What is the collision density? Solution:
A = 1cm 2
x = 0.1cm N = 0.048 × 10 24 atoms / cm 3
σ t = 4.5b φ = 4 ×1010 n / cm 2 − sec Σ t = N σ t = 4.5 × 10 −24 × 0.048 × 10 24 = 0.216cm −1
Number of interactions per second (in entire target): Collision density:
F = Σ tφ = 0.864 × 1010
= Σ tφ Ax = 0.864 × 109
3.5) Calculate the mean free path of 1‐eV neutrons in graphite. The total cross‐section of carbon at this energy is 4.8b. Solution:
σ t = 4.8b for 1‐eV 24 3 From Table II.3, N = 0.08023 × 10 atoms / cm
⇒λ =
1 1 = = 2.5977cm Σt σ t N
3.7) A beam of 2‐MeV neutrons is incident from the left on a target that extends from x=0 to x=a. Derive an expression for the probability that a neutron in the beam will have its first collision in the second half of target that is in the region a/2