Bleem Neutron

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Discovering the NeutronLindsey Bleem

 April 23, 2008

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Outline

!Motivation

!Biography

!Preliminary Experiments!The Discovery

! Implications

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Motivation

! Rutherford:" “The existence of such

atoms seems almost necessary to explain thebuilding up of the nuclei of heavy elements; for unlesswe suppose the productionof charged particles of very high velocities it is difficult tosee how any positively charged particle can reach

the nucleus of a heavy atoms against it’s repulsivefield.” 

!  Atomic Mass - AtomicNumber data

http://www.gutenberg.org/files/20417/20417-h/images/image388a.jpg 

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Sir James Chadwick

! British scientist, born

1891 in Manchester 

England

! Student of Rutherford’s,PhD 1921 Cambridge

! 1932 discovered neutron

! 1935 Nobel Prize

!

Manhattan Project! Died 1974

http://www.aps.org/publications/apsnews/200705/physicshistory.cfm

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Preliminary Experiments

! 1930; Bothe & Becker " penetrating, non-ionizing

radiation from berylliumbombardment

"Presumed radiation wasgamma rays

! 1932: Joliot-Curie & Joliot

" paraffin exposed to thisradiation

"Observed high speedproton ejected (3e9 cm/s)

" result atomic recoilsinterpreted as Comptonscatter 

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Chadwick’s Objections:

! Two Key Objections:" theoretical predictions concerning scattering rates

"Energy principals

!Compton Scattering: E_gamma ~50MeV

!However, most energetically favorable scattering allows~14MeV for the recoiling proton

http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/neutrondis.htmhttp://www.chemcases.com/nuclear/nc-01.html 

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Experimental Setup

! Evacuated vessel with polonium source and

beryllium target

! Ionization Chamber connected to amplifier and

oscillograph

J. Chadwick, PRSL,  A136 , 692, 1932.

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Data

J. Chadwick, PRSL,  A136 , 692, 1932.

http://nobelprize.org/nobel_prizes/physics/laureates/1935/chadwick-lecture.pdf 

Compton Scattering by Gamma Rays

would violate laws of conversation

and energy! 

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The Neutron

! Problem resolved if allow

for the existence of a

neutral particle of ~ the

same mass as the proton! Estimate particle mass by

combining information

from paraffin and nitrogen

recoils

!  Nuclear decaymeasurements yield finer 

mass estimates (1.0085)

http://nobelprize.org/nobel_prizes/physics/laureates/1935/chadwick-lecture.pdf 

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Further Study

! 1932 Neutron Structure: combined electron-proton

! By 1935 advancements included:

"neutron accepted as an “elementary” component"evidence for proton-neutron interactions governing

nuclear stability

"Beta decay and Pauli’s neutrino

! Scientists realized neutrons could be used toprobe atomic nuclei" lead to bombardment of Uranium with neutrons (Fermi)

"observation of nuclear fission

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Implications

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Conclusions

!Discovery of the neutron important

advancement in the study of particles

!Resolved open questions concerningnuclear structure and additionally provided

a new probe of the nucleus

!Opened up new fields of physics and

made possible the development of nuclear power and nuclear weapons

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References

! Chadwick, James. “The Neutron and its properties”.Nobel Lecture, Dec. 12, 1935.

! Chadwick, James. “The Existence of a Neutron”. PRSL,A136, 692, 1932.

! APS Physics. “This month in Physics History”, May2007.http://www.aps.org/publications/apsnews/200705/physicshistory.cfm

! Hyperphysics. “Discovery of the Neutron”.http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/neutrondis.html

! Chemcases.com. “Nuclear Chemistry Discovery of theNeutron” http://www.chemcases.com/nuclear/nc-01.htm