Download 13.2.1-8 Nuclear Physics

13.2.1-8 Nuclear Physics
 These notes were typed in association with Physics by Michael Dickinson. For further reading and
explanation see:Physics, Tsokos (purple): Ch 6.4 , Physics, Giancoli (mountain): Ch 27
13.2.1 Explain how the radii of nuclei may be estimated from charged particle scattering experiments.
 Remember back to the gold foil experiment!
 Some of the alpha particles will be deflected because of
electrostatic repulsion.
 We can use this to estimate the size of a nucleus.
 If an alpha particle is fired exactly at the center of a
nucleus, its velocity will reduce as its kinetic energy is
converted into electrostatic potential energy.
 Eventually the alpha particle’s velocity reaches zero.
 Then it is projected back from where it came from.
 This is all due to electrostatic repulsive forces.
 It’s very similar to the motion of a ball thrown vertically
into the air.
 Kinetic energy is converted to gravitational potential
energy
 Kinetic Energy before = Electrostatic Potential Energy
at closest approach
 E = W = Fs =Fr (r distance between
 Felectric = (kq1q2)/ r2
 E = ((kq1q2)/ r2 ) x r
 E = (kq1q2)/ r
Example B
 E = (kq1q2)/ r
 What is the distance of closest approach for an alpha particle (charge = +2e) aimed at a gold foil (charge
= +79e target). The alpah particle has an initial kinetic energy of 5MeV (or 8 x 10-13J).
 Answer: 4.55 x 10-14m
 It is assumed that the alpha particle never actually touches the nucleus.
 The more initial energy given to the particle the closer it will get before it is repelled.
 That’s how we know the size is around 10-15m
Practice 9
 Calculate the distance of closest approach for alpha particles aimed at a piece of gold foil with and initial
kinetic of 8.2MeV.
 Answer: 2.7 x 10 -14m
13.2.2 Describe how the masses of nuclei may be determined using a Bainbridge mass spectrometer.
 ***Special Note - Students should be able to draw a schematic diagram of the Bainbridge mass
spectrometer, but the experimental details are not required Students should appreciate that nuclear mass
values provide evidence for the existence of isotopes.
 This stuff can get kind of confusing. First I want to you watch the video and then we’ll talk about it.
http://www.youtube.com/watch?v=eZnd_gyTwuE&list=PL80C5AF536A5A90DF
 First: Source of ion
 Second: Two slits
 This focuses them into a “beam” or straight line.
 Third: Velocity selector
 An electric field and magnetic field are set perpendicular to one
another.
 Allows only ions with specific velocities to pass.
 Those that are to fast or slow will be pulled to either plate.
 Electric force = magnetic force
 qE = qvB1
 v = qE/qB1
 v = E/B1
 Fourth: Second magnetic field
 Produces a centripetal force on ions.
 Centripetal force = magnetic force
 mv2/r = qvB2
 m = qvB2 r / v2
 m = qB2r / v
 Fifth: Ions hit detection plate
 Since they all have the same speed and charge the only difference
is their mass.
 Heavier isotopes will produce larger radius
 Lighter isotopes will produce smaller radius
 Substitute the velocities out and you get:
 v = E/B1
 m = qB2B1 / E
 13.2.3 Describe one piece of evidence for the existence of nuclear energy levels. .
13.2.3 Describe one piece of evidence for the existence of nuclear energy levels.
 When atoms undergo radioactive decay the energy of the decay particle are actually discreet. Meaning they
have specific levels.
Example
 Radium-226 to Radium-222, an alpha particle is released.
 We can observe that the energy carried by the alpha particle can be either 4.59MeV OR 4.78MeV