Murray hill New Jersey during the period 1972 1982 Loren Pfeiffer Then at Bell Labs now at Princeton University Princeton NJ This is Rajus first paper with me We showed that Heisenbergs uncertainty principle ID: 777762
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Slide1
Raju Raghavan’s early days at Bell Laboratories (Murray hill, New Jersey during the period 1972 – 1982)
Loren Pfeiffer
Then at Bell Labs, now at Princeton University, Princeton, NJ
Slide2This is
Raju’s
first paper with me:
We showed that
Heisenberg’s uncertainty principle was not violated in a Au197 Mössbauer experiment, as had been suggested in an earlier PRL.
Slide3You will need some background to appreciate this paper:Raju and Primila
Raghavan had just arrived
(from Purdue and MIT via a stint in Germany) as Members of Staff at Bell Labs in the early 1970s. I worked on Mössbauer Effect research and Raju and I talked daily at lunch about the various new results coming out in Physical Review Letters using the
Mössbauer Effect.Here are pictures of Raju and me taken at Bell Labs in the early 1970s:
Slide4Background on the Mössbauer Effect:
Rudolf
Mössbauer won the Physics Nobel Prize in 1961 for demonstrating resonant nuclear
γ-ray scattering without nuclear recoil. From the middle 1960s to early 1970s Mössbauer scattering was a very hot topic
in physics.
Fe57
Fe
57
Co
57
s
table Fe
57
β
-decay
excited Fe
57
#1. Radioactive parent decays to excited Fe
57
nuclear state.
#2.
excited
Fe
57
emits
γ
-ray
decaying to stable Fe
57
ground-state
.
#3. Fe
57
in absorber
resonantly absorbs
γ
-ray.
#4. Fe
57
in absorber
re-emits γ-ray in some random direction.
The problem: These excited states are so sharp that the energy losses from the recoiling nucleus during the γ-ray emission and absorption would spoil the resonance.
Mössbauer showed: the nuclear recoil during the γ-ray emission and absorption could be taken up by the chemical bonds in the crystal lattice. Recoilless scattering!
γ-ray
γ
-ray
Slide5In a M
ӧ
ssbauer
experiment we mostly are concerned with steps #2, #3, and #4.
γ-rays
γ
-ray detector
We move the absorber back and forth, adding or subtracting a small Doppler velocity to shift the
γ
-ray energy in and out of nuclear resonance.
γ
-ray
M
ӧ
ssbauer
source
M
ӧ
ssbauer
absorber
As the
γ
-ray energy is varied in the experiment,
we expect a small dip in the detected counts at exact nuclear resonance
, because
γ
-rays absorbed in the absorber are likely re-emitted away from the detector.
Slide6The extreme sharpness of the nuclear resonant levels is what made Mössbauer research so compelling.
The
energy width
ΔE of the nuclear resonant level is given by the nuclear decay lifetime Δt
and the Heisenberg Uncertainty Principle: Δt ΔE ≥ ħ/2. Thus if we measure Δt, we can calculate ΔE,
ΔE ≥ ħ/2
Δ
t .
For Au
197
the measured nuclear lifetime is 1.95±.06
nsec
,
Thus Heisenberg’s relation implies
Δ
E = 0.923 ± .006 mm/sec
.
A Mossbauer experiment involves
two
nuclear
lifetimes,
one for the source and one for the absorber, so the
minimum expected Mossbauer width is 2ΔE =
1.85 ± .012 mm/sec.
But in October 1972 a Physical Review Letter was published by Gilbert
Perlow and his postdoc suggesting for certain Au197 compounds that the measured
linewidth 2Δ
E is only 1.54 mm/sec, thus implying a violation
of Heisenberg’s uncertainty relation.
Slide7Dr.
Perlow
was a highly respected physicist at Argonne National Laboratories. He was at the time Principal Editor of the journal, Applied Physics Letters.
Slide8In late 1972
Raju
and I did an exhaustive series of
Mössbauer
measurements on the relevant Au197 compounds. We found no anomalous linewidths and said so in this 1973 paper.In 1974 I published a paper showing how the least-squares
fitting program that most physicists were using contained a bias which assigned a too small
an error ba
r to a
Lorentzian
line having a width defined by only a few channels.
The narrow
linewidth
effect goes away if the data set is modified to incorporate more channels.
Our measured
linewidth
equals the Uncertainty
linewidth
.
Perlow
PRL paper
linewidth
Slide9The next paper that
Raju
and I wrote announced the discovery of the
Mössbauer
Effect in a new isotope: Ge73
.Raju
came to Bell Labs with an encyclopedic knowledge of the nuclear isotopes.
He
had been keeping a list of possible
Mössbauer
Effect isotopes in his pocket since his early days at the Tata Institute in India.
Ge
73
was
the most attractive candidate on that list.
Its 3
sec
lifetime
made it potentially
the
sharpest
Mössbauer resonance. The
Ge73 experiment had never been done because of difficult x-ray background problems, but with care and a modern cooled Si
γ-ray detector we saw a clear nuclear resonance signal in a room temperature experiment.
Slide10Several years later I was able to further improve the Ge73 Mossbauer signal and see the resonance at the natural
linewidth
expected from Heisenberg’s uncertainty principle.
Using a
high purity natural
Ge
single crystal absorber NOT enriched in the rare
7% natural abundant isotope Ge
73
.
The M
ӧ
ssbauer
resonance is indeed sharp, but the signal is small because the isotope Ge
73
has a natural abundance of only 7%.
Slide11Also for this paper I grew by Molecular Beam
Epitaxy
(MBE) a
special
Ge single crystal absorber using 2 grams of very expensive germanium isotopically enriched from 7% to 90% in Ge73.This enriched absorber brought the Ge73 signal to a huge 6% effect.
This special Mӧ
ssbauer
crystal was my first foray into
M
olecular
B
eam
E
pitaxy
.
After 1987 I would spend the next 25 years doing MBE crystal growth.
Thus
Raju’s
ideas and enthusiasms in the 1970s had a profound effect on my entire later career.
Slide12Molecular Beam Epitaxial growth of an enriched Ge
73
single crystal
Natural
Ge
single crystal (high purity)
7.7% Ge
73
9
7% Ge
73
single crystal
9
7% Ge
73
Powder
Slide13The
extreme difficulty
of the
Ge
73 experiment was an omen that the Mossbauer Effect fever that had gripped physics was cooling down.
The easy experiments had all been done!
In the mid 1970s
Raju
turned his encyclopedic knowledge of nuclear isotopes loose on a new problem:
He proposed to build a new higher quality
detector
that would be
uniquely sensitive to neutrinos from the sun
.
This was a truly elegant scheme for detecting solar neutrinos!
A neutrino capture by In
115
would give a
signature
of
two coincident
γ
-rays each with well defined energies
.
And the threshold for solar neutrino capture was only 128 KeV.
n
eutrino capture
Slide14The only problem is that In115 is itself very slightly radioactive! In115
is 95% naturally abundant, but
not
stable, its radioactive half life is about 5 x 1014 years.This would clearly be the main source of background for
Raju’s solar neutrino detector, and this background had to be evaluated if his detector idea was to succeed. Soon enough Raju’s enthusiasm for his In115 solar neutrino detector proposal inspired Allen Mills and me to build a small scale In115
liquid scintillator in one corner of Allen’s lab. We enlisted the organic chemist, Ed Chandross, to build an organic scintillator heavily loaded with In
115
, and surrounded that with about 1000
Kgm
of lead shielding.
The Bell Labs plant department gave Allen and me some grief about the floor loading from the pile of lead bricks in Allen’s third floor lab.
Slide15In this paper Allen and I measured the radioactive background be to expected in
Raju’s
solar neutrino detector, and also measured the
halflife
of In115 to an accuracy of 6%.The In115 half-life is about 10,000 times longer than the age of the universe since the big bang!
Slide16I will close by calling your attention to an article that
Raju
Raghavan
wrote for Physics Today in 1984 as part of the celebration of the 25th anniversary of the discovery of the Mӧssbauer Effect.It tells about his excitement as a young student in India when the news of the Mossbauer Effect broke in 1959. For a few months anyone with a good idea and some simple equipment could do first-class physics research .
Every time I read this article I am reminded again that Raju
had a wonderful literary style as well as a deep love for physics research.
Slide17Raju
recounts the origins of his detailed list of M
ӧ
ssbauer
candidate isotopes.
Slide18Raju’s
admiration for his boss
Jha’s
knowledge of
“every isotope on the nuclide chart” reflected Raju’s own deep reservoir of knowledge on this subject.
Slide19Raju had a deep love for physics research.
He had more good ideas than most of us physicists are ever blessed with.
And when a good idea did occur to him, he had such an infectious enthusiasm that he could inspire others to become involved in his quest.
He was a wonderful colleague and friend.
My memories of Raju Raghavan: