Harvard, Jefferson Tower

1
Harvard, Jefferson Tower

• Photons were moving in a tower 74 high.
• If ? quantum was emitted from the top, blue
  shift was observed on the bottom
• If ? quantum was emitted from bottom, on a top a
  red shift was observed

2
The basis elements

• The carefully prepared source and absorber of
  photons (?-rays with energy 14.4keV) were nuclei
  of isotope 57Fe.

3
What are we waiting to observe for?
We expect the ? ray detector at the top of the
tower to measure a lower frequency and vice
versa. Lets estimate expectation value for
frequency shift! The magnitude of the expected
shift can be obtained from the conservation of
energy and the relativistic mass-energy
equivalence.
4
Effect estimation
It seems plausible to assume that for shift
detection, ?E should have the same order of
magnitude as the natural absorption line width .
This accuracy could be achieved owing to the
Mössbauer effect, due to which photons lines are
extremely narrow Nevertheless, the absorption
line is not narrow enough for direct measurement
of red shift effect.
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Effect estimation
For 57Fe we have a line width at half-height
point
Pound Rebka used a known experimental method to
solve this problem
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Resolution improving technique

• The modulation technique is based on the Doppler
  effect
• Essential element of this technique is a
  mechanical motion with precisely controlled
  velocity

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Resolution improving technique (cont.)
Combining data from two periods having Doppler
shift of equal magnitude, but opposite sign,
allowed measurement of both sensitivity and
relative frequency shift
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Resolution improving the speed of modulation
estimation
The absorber power spectrum line had a Lorenzian
shape Width at half-height point
Substitution of the numbers gives an oscillating
source motion speed of
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Resolution improving error estimation
Furthermore, the mentioned modulation technique
allowed Pound Rebka to observe the hyperfine
structure of 57Fe
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Null measurement
The gravitational shift could then in principle
be deduced from the difference in the two
counting rates. It is advantageous however to
provide an additional superposed constant
velocity motion chosen so as to just compensate
for the red shift, thus allowing us to make a
null measurement. The required velocity was
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Temperature error estimation

• The second order Doppler shift resulting from
  lattice vibrations required that the temperature
  difference between the source and absorber be
  controlled and monitored.

A difference of 0.6 0C would produce a shift as
large as the whole effect observed.
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Temperature error estimation
The difference of the shift seen with ? rays
rising and that with ? rays falling should be the
result of gravity. The average for the two
directions of travel measured an effective shift,
resulting due to other reason, which is about
four times larger than the shift caused by
gravity!
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What is the cause of mean shift?

• By additional experiment Pound and Rebka
  confirmed their assumption that this shift was an
  inherent property of the particular combination
  of source and absorber. They measured the
  inherent shift for each absorber unit when it was
  six inches from source and explained it by
  different Debye temperatures.

14
A block diagram of the experimental arrangement
Where the plus sign indicates that the frequency
increases in falling
15
Acknowledgments
Yulia Preezant is like to thank Mr. Yevgeni
Preezant (Technion) for his helpful advice and
translation oral presentation in Hebrew.
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References

1. R.V.Pound and G.A.Rebka,Jr., Phys.Rev.Letters
   4,337(1960)
2. Gunther Wertheim, Mössbauer Effect principles
   and applications, Academic press, New York ,1964
3. R.L. Mössbauer, Nobel Lecture,December11,1961
4. R.V.Pound and G.A.Rebka,Jr., Phys.Rev.Letters
   3,554 (1959)
5. R.V.Pound and G.A.Rebka,Jr., Phys.Rev.Letters
   3,439 (1959)
6. L.B. Okun, K.G. Selivanov, V.L. Telegdi,
   Usp.Fiz. Nauk 42,1045 (1999)