rutherford scattering edan bainglass jose chavez kennedy izuagbe
TRANSCRIPT
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…
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. Thomson
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 EXPERIMENTS
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 matter
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°
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. Rutherford
In 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”
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
b
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
𝑦= 𝑄𝑛𝑡𝑏2
16 r2 ∙ sin4𝜑2
or𝑦=𝑄𝑛𝑡
16 ( 𝑒2
4𝜋𝜖0 )2 𝑍1
2𝑍 22
𝑟2𝐾2 sin4𝜑2
y = number of scattered particles
Q = number of incident particles
n = atoms per unit volume
t = target thickness
Z1 = atomic number of alpha
particle
Z2 = 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 4th 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)
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 velocity
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
conditions
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)
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 world
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).