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Raju   Raghavan’s  early days                           at Bell Laboratories Raju   Raghavan’s  early days                           at Bell Laboratories

Raju Raghavan’s early days at Bell Laboratories - PowerPoint Presentation

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Raju Raghavan’s early days at Bell Laboratories - PPT Presentation

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

ray ssbauer raju nuclear ssbauer ray nuclear raju effect absorber crystal physics detector paper raju

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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

Slide2

This 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.

Slide3

You 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:

Slide4

Background 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

Slide5

In 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.

Slide6

The 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.

Slide7

Dr.

Perlow

was a highly respected physicist at Argonne National Laboratories. He was at the time Principal Editor of the journal, Applied Physics Letters.

Slide8

In 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

Slide9

The 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.

Slide10

Several 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%.

Slide11

Also 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.

Slide12

Molecular 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

Slide13

The

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

Slide14

The 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.

Slide15

In 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!

Slide16

I 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.

Slide17

Raju

recounts the origins of his detailed list of M

ӧ

ssbauer

candidate isotopes.

Slide18

Raju’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.

Slide19

Raju 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: