Introduction toNeutron Physics

Carlo Cazzanigacarlo.cazzaniga@mib.infn.it

Liceo Scientifico Vittorio Veneto

Can you see a neutron? Neutrons do not feel electromagnetic force ( q

n= 0 )

Neutrons feel nuclear forces (strong and weak interaction)

Knock-on effect:

n p

n'

p'

θ

φ

Ep' = E

n cos2θ

You detect secondary charged particles!

Discovery of the Neutron (1932)

Open questions before the discovery:

How to explain isotops?

How to explain nuclear spin?

Chadwick's experiment:

Protons up to 5.7 MeV

Neutron Beta Decay

The neutron and the proton are not elementary particles → made by quarks

- The neutron is no longer stable outside of the nucleus (T=0.25h)- Neutron can transform into a proton- n and p masses are a bit different.

Enrico Fermi and ”The via Panisperna boys”

● 1926. Fermi get the chair of Theoretical Physics.

● From 1929, Fermi and Corbino dedicated themselves to the transformation of the institute into a modern research centre.

● Studies of Atomic Spectroscopy

● Studies of the interaction of neutrons with all known elements (nuclear reactions).

● 1934. ”Discovery” of slow neutrons.

● 1938. Due to political situation in Italy and Europe, the group dispersed and most of its members

emigrated. Left to right: D'Agostino, Segrè, Amaldi, Rasetti and Fermi

Fission and other neutron-nuclei interactions

n + AX

fission

neutron capture

neutron scattering

alpha reaction

Etc.

Reaction

channels

There are several reaction channels

It's a matter of probability

Why slow neutrons for fission?”As big as a barn door”

Nuclear Fission

1 barn = 10-24 cm2

Cross section = probability (to have fission)

235U vs 238U

Natural Uranium:- 0.7% U235- 99.3% U238

Enrichment!

Building the reactor and the bomb”The Manhattan Project”

Nuclear Physics becomes a national priority

New laboratories open (Los Alamos, Oak Ridge, Chicago)

Hiring of the best scientists

Enrico FermiLawrence, Seaborg and Oppenheimer

Neutron Cycle

The k parameter is the multiplication factor defined as the ratio of « useful » neutrons produced in average per fission of one generation to the number of « useful » neutrons of the previous generation.

( ) ( ) ( )n t n

k t=

−"

#$$

%

&''

01

exp

k>>1 → bomb(super-critical)

k≈1 → controlled reactor (critical)

How to get critical (k≈1)?The moderator material

Remember:- Slow Neutrons are best for fission- Hydrogen (light atoms) can slow neutrons down

Chicago Pile, first controlled nuclear reactor

Uranium and Graphyte Layers

Coded message:

”The italian navigator has landed the new world””How are the natives?””Very Friendly”

How to get supercritical (k>>1)?High Enrichment

or Plutonium ProductionRemember:- U235 is best for fission than U238

- Enrichment is a very difficult task- Plutonium production needs a reactor and has problems of radioactive hazard

Trinity BombThe first atomic bomb is readied for testing near Alamogordo, New Mexico, in July 1945.

”Little Boy” and ”Fat Man”

2 sub-critical pieces of U235

Plutonium core

TNT explosive

Little Boy (Hiroshima)Gun-type assembly method

Fat Man (Nagasaki)Implosion assembly method

The key point is to have a configuration which is sub-critical (k<1) before the bomb is fired, and super-critical (k>1) after.

The decision to drop the bomb

Was it possible to have the project stopped after the germans were defeated?

Could the military use of the bomb be avoided?

Physicists were asked to write a report about the possible application of the bomb:

"The opinions of our scientific colleagues on the initial use of these weapons... range from the proposal of a purely technical demonstration to that of the military application best designed to induce surrender. […] We find ourselves closer to these latter views; we can propose no technical demonstration likely to bring an end to the war; we can see no alternative to direct military use."

Sir Winston Churchill about the decisions that were taken:

”Who reads the story of that complicated period could tend to severely judge people and things. He may forget that those people had to face problems which the good reader has never had to solve”

In a 1969 interview, Chadwick recalled:

"I realized then that a nuclear bomb was not only possible-- it was inevitable... and there was nobody to talk to about it. I had many sleepless nights. But I did realize how very serious it could be. And I had then to start taking sleeping pills. It was the only remedy. I've never stopped since then. It's 28 years, and I don't think I've missed a single night in all those 28 years."

Where does this energy come from?

Nuclei at the top of the curve are more stable. So Heavy nuclei want to split, light nuclei want to join together. The difference in binding energy is

reliased.

E=mc2

Electric power from nuclear reactors

Nuclear energy → thermal energy → kinetic energy → electric energy

- Thermodynamic cycle (steam cycle) to transform thermal energy into kinetic energy.

- Generator (Faraday induction law) to transform kinetic energy into electic energy

Nuclear fusion

Tritium is bred from lithium using the neutron6Li + n → 4He + T

D + T → 4He + n

Creating conditions for fusion

Gravitational Confinement

Heating: compression (gravity), nuclear reactions

Magnetic Confinement

Heating: Ohmic (by electric currents), Microwaves, Particles beams, nuclear reactions

Inertial Confinement

Heating: compression (driven by a Laser) nuclear reactions

The TOKAMAK

Main next step: ITERInternational Thermonuclear Experimental

Reactor

The goal is to achieve a positive energy gain. ITER will be operaing from 2020.

Neutron Physics on 2011 call!

Neutrons from fusion New generation fission reactors (fast reactors)

Solid state structures Neutron sources

HintsBooks

”Atoms in the family” by Laura Fermi

"Surely you're joking, Mr. Feynman!": adventures of a curious character. By Richard Feynman

Watch on YouTube Super quark, documentary completly dedicated to Enrico Fermi (ITA):

https://www.youtube.com/watch?v=h8qP6MxdTAM&list=FLqsLhf6ftZjF4zJ112u6P2Q&feature=mh_lolz

Feynman, nobel laureate, talks about his experience in Los Alamos:

http://www.youtube.com/watch?v=_ah7f-1M2Sg

Steven Cowley, director of the UK leading fusion research center, talks about nuclear fusion on TV:

https://www.youtube.com/watch?v=S6BLFdBfgfU&feature=player_embedded

Inertial confinement fusion on BBC "Horizon" :

http://www.youtube.com/watch?v=DyB7Ho_W9RE

Robert Goldston (Princeton Plasma Physics Lab) explains fusion:

https://www.youtube.com/watch?v=vpOmkv8oM4k

Movie ”I ragazzi di via Panisperna”, by G.Amelio (ITA)

That's all folks!