Edan Bainglass Jose Chavez Kennedy Izuagbe After his discovery of alpha particle emissions from a radioactive isotope of Radon gas for which he was awarded the 1898 Nobel prize in Chemistry Rutherford spent a considerable amount of time and effort into their investigation ID: 480619
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Slide1
RUTHERFORD SCATTERING
Edan Bainglass
Jose Chavez
Kennedy IzuagbeSlide2
After his discovery of alpha particle emissions from a radioactive isotope of Radon gas, for which he was awarded the 1898 Nobel prize in Chemistry, Rutherford spent a considerable amount of time and effort into their investigation
As a student of J.J. Thomson at Cambridge University, Rutherford was familiar with the Plum Pudding model and used it as the standard for his investigations
How it all started…Slide3
The
“Plum pudding” model
Shortly after discovering the electron in 1897, Prof. J.J.
Thomson proposed the “Plum Pudding”
model
“We suppose that the atom consists of a number
of corpuscles moving about in a sphere of uniform positive electrification” – J.J. ThomsonSlide4
Rutherford
initially failed in counting individual
alpha particles
Upon moving to the University of Manchester, he teamed up with Dr. Hans Geiger, and with the use of Geiger’s apparatus, they began recording individual scintillations of
alpha particles
The initial experimentsSlide5
Strange results
The assumption, based
on the Plum Pudding model,
was that the alpha particles will mostly pass right through the atom, with minimal deflection
Geiger
originally calculated the most probable deviation of alpha particles to be
within 2°Geiger suggested that the experiments should be given to a young student named Ernest Marsden
Marsden returned to Geiger shortly after conducting the experiment with strange results – some of the alpha particles had been deflected at a
“considerable”
angle
Rutherford
instructed Geiger and Marsden to perform further investigation
into the matterSlide6
Geiger and Marsden's 1909 Experiment
Geiger’s Scintillation Method
Radium was used as a powerful, continuous source of alpha particles
A low powered microscope was used to observe scintillations on a ZnS screen
Different reflecting materials were used
Alpha particles
were reflected unto the ZnS screen regardless of the angle of incidence About 1 in 8000
particles were reflected at angles
greater than 90
°Slide7
Rutherford
'
s Theory – The nucleus
Rutherford was shocked!!!
“It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.“
–
E. RutherfordIn order to explain such events, Rutherford proposed a new atomic model – one with a massive central charge confined to a very small volume surrounded by an opposite and equal charge uniformly distributed across the remainder of the atom
Rutherford later dubbed the central charge “the nucleus”Slide8
Rutherford
'
s Theory (Cont.)
Rutherford’s model explained both small and large deflections
As the ratio of the impact parameter p to the instantaneous distance from the nucleus b decreases, the deflection angle
φ
increases – stronger coulomb force (
)
Rutherford suggested that such large deflection angles most likely occurred due to
a single scattering event
p/b
10
5
2
1
.5
.25
.125
φ
5°.7
11°.4
28°
53°
90°
127°152°
φ
bSlide9
Rutherford
'
s Theory (Cont.)
Rutherford showed that the number of alpha particles scattered per unit area into the detector at scattering angle
φ
is given by
or
y = number of scattered particles
Q = number of incident particles
n = atoms per unit volume
t = target thickness
Z
1
= atomic number of alpha particle
Z
2
= atomic number of target nucleus
K = kinetic energy of incident particle
Φ
= scattering angle
Rutherford noted that the number of scattered particles is proportional to
The inverse square of the kinetic energy of the incident particle
The inverse
4
th
power of the sine of half the deflection angle
The square of the atomic number of the nucleus
The thickness of the target (for thin foils)Slide10
Experimental Proof (1913)
Geiger and Marsden went on to prove their professor’s theory. They tackled his four main conclusions by investigating the change of the number of scattered particles with:
Variation of angle
Variation of thickness
Variation of atomic weight
Variation of velocitySlide11
The equipmentSlide12
in good agreement
Geiger and Marsden found Rutherford’s theory to be correct
“It may be mentioned in anticipation that all the results of our investigation are in good agreement with the theoretical deductions of Prof. Rutherford, and afford strong evidence of the correctness of the underlying assumption that an atom contains a strong charge at the
center
of dimensions, small compared with the diameter of the atom
”
– Geiger and Marsden (1913)They concluded that it would be possible to calculate the probability of an alpha particle being scattered through any angle under any specified conditionsSlide13
Where did we go from there?
Rutherford’s theory and subsequent experiments provided a stepping stone for future research into the structure of the atom
A few examples of such research:
Bohr’s stationary quantized energy states (1912) describing the electron structure of the atom (later adjusted by quantum theory)
Rutherford’s discovery of the proton (1920
)
The discovery of the neutron by James Chadwick (1932)The discovery of quarks (1968, 1974, 1977, 1995)Slide14
Conclusions
Rutherford’s work was invaluable to science and allowed us to have a clearer picture of the inner workings of our world
It is interesting to note that similar to the strange results of his team, Rutherford turned out to be quite the anomaly, as his greatest achievements – the theory of the nucleus, the discovery of the proton – all came after he had already been awarded the Nobel prize - a first!
The methods used by Rutherford and his team are still used today to further investigate the atomic worldSlide15
references
1. E. Rutherford, F.R.S.*, The Scattering of
and β Particles by Matter and the Structure of the Atom, Philosophical Magazine.
Series 6 vol. 21, p. 669-688 (1911).2. H. Geiger and E. Marsden, On a Diffuse Reflection of the Particles. (1909).3. H. Geiger and E. Marsden, Assistants Paper. Philosophical Magazine 25. p 605-623 (1913
).
4.
J.J. Thomson, On the Structure of the Atom: an Investigation of the Stability and Periods of Oscillation of a number of Corpuscles arranged at equal intervals around the Circumference of a Circle; with Application of the Results to the Theory of Atomic Structure. Philosophical Magazine. Series 6, Volume 7, Number 39. p. 237-265. (1904).