Nuclear Physics 1 st of all, It is pronounced “new-clear” not “new-que-ler” Physics 100 Chapt 25
Dec 20, 2015
Nucleons
proton neutron
mass 1.672x10-27kg 1.675x10-27kg
mc2 938.27 MeV 939.57 MeV
Charge +1.6x10-19 C 0
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Nuclear force
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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
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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)
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Eventually, the electrical repulsionoverwhelms the nuclear attraction
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
KE = Mparentc2 – (Mdaughters c2)
-particle Kinetic Energy
Some mass is changed into Kinetic Energy
All the a particles havethe same Kinetic Energy
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??
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