Tutorial Answers - School of Physics - The University of New South ...

(b)
98
43
Tc→
98
42
+
Mo + β + ν
e
Q = ( M
P
− M
D
i.
e.
> 0∴Yes
− 2m
) c
e
2
= (97.907215 − 97.905407 − 2 × 5.485803×
10
−4
) u × 931.5MeV
/ u = 0.662MeV
(c)
e
+
Tc→
− 98 98
43 42
Mo + ν
e
2
Q = ( M − M ) c = (97.907215 − 97.905407) u × 931.5MeV
/ u = 1.684MeV
i.
e.
> 0∴Yes
P
D
21. [S 46.31]. A 2 MeV neutron is emitted in a fission reactor. If it loses half of its kinetic
energy in each collision with a moderator atom, how many collisions must it undergo to achieve
thermal energy (0.039 eV) ?
2
n
6
2 × 10
7
= = 5.13×
10 ∴ n = 25.6 → 26
0.039
22. [S 46.37]. Assume a deuteron and a triton are at rest when they fuse according to
d + t → α + n . This reaction has a Q value of 17.6 MeV. Determine the kinetic energies
acquired by the α and the n.
p
2 2
pα
pn
pn & + = 17.6MeV
= Kα
+ K n → K = 3.5MeV
& K n = 14. 1MeV
2m
2m
α = α
α n
23. [Rohlf 11.15]. If the activity of a substance drops by a factor of 32 in 5 seconds, what is its
half-life ? 32 5
= 2 ∴5s = 5×
t 1 ∴t
1 = 1s
2 2
24. [R 11.26]. Show that the decay n → p + e cannot conserve angular momentum.
All particles have spin 1/2. Two spin-1/2s cannot be combined to yield s=1/2.
25. [R 17.16]. Which of the following strong interactions are allowed ? If a process is
forbidden, state the reason.
0
p + p → π
−
π + p → K
+
p + p → π
0
p + p → π
K
−
+ n
0
+ p → π + Λ
0
+ n
+ n + n
+
+ π
−
+ π
0
−
Electric Charge Baryon # Strangeness
Yes No Yes
Yes Yes No
No Yes Yes
Yes Yes Yes
Yes Yes Yes