Nuclear Physics Micro-world Macro-world Lecture 16.
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Transcript of Nuclear Physics Micro-world Macro-world Lecture 16.
Nuclear Physics
Micro-world Macro-world
Lecture 16
Neutron
By 1930, the charge and masses of many differentnuclei were known. The charges were found to be Qnucleus = Z x proton charge & the masses wereMnucleus = A x proton mass
Z is always an integer A is nearly an integer
But, except for hydrogen where A=Z=1,A & Z are different.
Electrons in the nucleus?
One idea was that the nucleus contained A protonsand A-Z electrons.
But Heisenberg’s uncertainty principle forbids this:
Suppose x= 1x10-14m typical size of a nucleus
px > 6.6x10-34Js1x10-14m
= 6.6x10-28kgm/s
vx > px me
6.6x10-28Js9x10-31kg
= >speed of light
non-relativistic calculationis invalid
Correct calculation gives Eelectron > 2x106 eV,such electrons would not stay inside a nucleus
Discovery of the neutron
James Chadwick
Po
Be
n detector
detectorsource
No electric charge; mass ≈ proton mass
Nucleons (protons & neutrons)
proton neutron
mass 1.672x10-27kg 1.675x10-27kg
mc2 938.27 MeV 939.57 MeV
Charge +1.6x10-19 C 0
+
Isotopes
Same chemistry; very different nuclear physics
Light isotopes
AX
Chemical symbol
A = number ofprotons + neutrons
ZZ=# of protons
All elements have isotopes
238
92U
235
92U
Nuclear force
++
The very powerful electrical repulsion (100’s of Newtons)
must be counteractedby some other verystrong attractive
force
This new force is called the Strong Nuclear force
Strong Nuclear force
Forc
e
Distance (x10-15m) 1.0 2.0 3.0 4.0 5.0
Attractive nuclear force
repulsive electrical force
attraction isstronger here
repulsion isstronger here
Strong Nuclear Force
• It is very strong– It overcomes the electrical
repulsion between positively charged protons that are only 10-
15m apart.
• It acts over a very short range– It is not felt by nucleons when they
are more than 10-15m apart.
• It is selective– It is felt by neutrons & protons, but
not by electrons
Nuclei can’t be too large
+++
++
+
+
+
+
+
+
++
++
A proton feelselectrical repulsionfrom every protonin the nucleus
It feels a Strong Nuclearattraction only to nearby nucleons
In larger nuclei, the electricalforce is bigger while the nuclear force stays the same.
There are no stable nuclei above Z=82 (Lead)
+
+
++
++
+++
Eventually, the electrical repulsionoverwhelms the nuclear attraction
Discovery of radioactivity
Different types of radiation
Marie Curie
---
+++
neutral
- charged
+ charged
decayZY Z-2X + 2A A-4 4
Conservation of energy in decay
ZY Z-2X + 2A A-4 4
parent daughters
Energy of parent= Energy of daughters
Conserv. of energy in decay
ZY Z-2X + 2A A-4 4
parent daughters
v
= MXc2 + Mc2 + KEMYc2KE = MYc2 - MXc2 - Mc2
Energy balancepar
ent d
aughte
rs
the mass difference,times c2, becomes KE
KE = Mparentc2 – (Mdaughters c2)
-particle Kinetic Energy
Some mass is changed into Kinetic Energy
All the a particles havethe same Kinetic Energy
ZY Z+1X + e-A A
n p + e-
Beta () decay
+ e-
6C147N14
+
++
+
++ +
++
+
+
++7p
7n6p8n
88Ra 89Ac + e-228 228
Other beta decays
1H 2He + e-3 3
Tritium Beta decay
+ e-
3H 3He+ +
+
Energy balance in beta decay
14C160 keV
14N+e-
1 keV = 1000 eV
Electron energy in 14C beta decay
160 keV KEe-
KEe-
None of the electrons have160 keV of kinetic energy they all have less than that amount
What has happened to the “missing” energy??
Pauli’s “desperate solution”
no chargeno mass
Another “unseen” particle isemitted in decay process
6C 7N + e- + 14 14
14C decay revisited
e-6C14
7N14
1H 2He + e- + 3 3
3H decay revisited
e-3H 3He
Energy balance in beta decay
parent KEe- + KE
daughters
Cosmic gallNEUTRINOS, they are very small. They have no charge and have no mass And do not interact at all. The earth is just a silly ball To them, through which they simply pass, Like dustmaids down a drafty hall Or photons through a sheet of glass. They snub the most exquisite gas, Ignore the most substantial wall, Cold shoulder steel and sounding brass, Insult the stallion in his stall, And scorning barriers of class, Infiltrate you and me! Like tall and painless guillotines, they fall Down through our heads into the grass. At night, they enter at Nepal and pierce the lover and his lass From underneath the bed-you call It wonderful; I call it crass.
John Updike
Radioactive half-life
N = N0 ( ½ )n
Number ofremaining atoms
Initial numberof atoms number
of halflives
Some half-lives
3H (3He + e- + ) 12.3 yrs14C(14N + e- +) 5730 yrs238U(234Th +) 4.5x109 yrs235U(231Th + ) 7.1x108 yrs226Ra(222Rn + ) 1600 yrs28Mg(28Al + e- + ) 21 hrs213Po(209Pb + ) 4x10-6 s
T1/2
Carbon-14 dating
14C content
Shroud of Turin