discovering the neutron - university of...
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Discovering the Neutron
Lindsey BleemApril 23, 2008
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Outline
MotivationBiographyPreliminary ExperimentsThe Discovery Implications
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Motivation
Rutherford: “The existence of such
atoms seems almostnecessary to explain thebuilding up of the nuclei ofheavy elements; for unlesswe suppose the productionof charged particles of veryhigh velocities it is difficult tosee how any positivelycharged particle can reachthe nucleus of a heavyatoms 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, born1891 in ManchesterEngland
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 this
radiationObserved high speed
proton ejected (3e9 cm/s) result atomic recoils
interpreted as Comptonscatter
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Chadwick’s Objections:
Two Key Objections: theoretical predictions concerning scattering ratesEnergy 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 andberyllium target
Ionization Chamber connected to amplifier andoscillograph
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 Rayswould violate laws of conversationand energy!
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The Neutron
Problem resolved if allowfor the existence of aneutral particle of ~ thesame mass as the proton
Estimate particle mass bycombining informationfrom paraffin and nitrogenrecoils
Nuclear decaymeasurements yield finermass 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” componentevidence for proton-neutron interactions governing
nuclear stabilityBeta 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 importantadvancement in the study of particles
Resolved open questions concerningnuclear structure and additionally provideda new probe of the nucleus
Opened up new fields of physics andmade possible the development of nuclearpower 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