From pre- ISOLDE to ISOLDE: some personal recollections
Torleif E. O. Ericson
Theory Group, Physics Department, CERN, CH-1211 Geneva, Switzerland
I have been asked to give some illustration of CERN in the early years as a background
to the ISOLDE decision on December 17th 1964.
Personally, I was somewhat involved at a late stage, but only on the sidelines.
Let me say from the beginning that in 1964 the very idea that the Radiochemistry
Group would generate one of the most successful and the most long-lived programs at
CERN would have been considered outright silly, not to say outrageous.
life.
The origins of ISOLDE go right back to the action of Niels Bohr in the last years of his
Immediately after the first 11 member states had agreed in 1952 to create CERN they
also agreed on the urgent need of a meson-producing accelerator of lower energy as a
training ground, which became the 600 MeV synchro-cyclotron. One also established
collaborations with some laboratories with relevant high-energy accelerators, in particular,
with the just finished 180 MeV synchrocyclotron at Uppsala, where accelerator expertise
was available. In the same year Niels Bohr persuaded the highly considered radiochemist
Alexis Pappas in Oslo to lead the CERN-related experiments at the Uppsala SC.
1952: Niels Bohr persuades Alexis Pappas from Oslo to do CERN-related radiochemistry at
the 180 MeV SC at Uppsala
When the SC entered operations in 1957 a radio-chemical group was needed for
many reasons. The head of the CERN SC program, Wolfgang Gentner, made a special trip to
Oslo to make Pappas accept the task to create such a group. It is possible that this choice
was influenced by Niels Bohr, who knew the capabilities of Pappas. That is how Gösta
Rudstam was selected to be the first head of the Nuclear Chemistry group in 1958. It also
explains why the key technical person on the later ISOLDE proposal, G. Andersson, comes
from the Uppsala group, which was highly experienced in isotope separator techniques.
At about the same time Pappas became an official advisor to CERN for such questions until
1967.
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Gösta Rudstam 1963
I think a word is needed about nuclear chemistry versus nuclear physics. After the
second world war, there was not much distinction between a nuclear chemist and an
experimental nuclear physicist for a couple of decades as I well remember from my early
years in low energy nuclear physics. But for the chemistry background the physics
problems investigated were mostly the same.
The role of the Nuclear Chemistry Group was primarily a support group but it studied
in addition mainly cross sections for spallation and fragmentation reactions at 600 MeV
and later also at 25 GeV.
The general mission of radiochemistry group was stated explicitly in a Memorandum
to Weisskopf 15th December 1961 by Gentner, Pappas, Rudstam; measurements of fission
and spallation cross sections at 600 MeV and 25 GeV to understand the mechanism of
complex processes and for various practical purposes of other experiments.
Rudstam was a recognized specialist in modeling of such reactions via simulations of
cascades using random selections of the paths. I remember on his desk a random number
generator consisting of a cylindrical prism that he rolled by hand. Metropolis at Los Alamos
developed a well-known similar model with the advantage of a far stronger computer
support of the Monte Carlo part.
When the SC came into operation in 1957 it was a world-class machine for the use of
the secondary low energy pions and muons. It was of course not in the running for higher
energies and its primary protons were not used much but for radiochemistry.
The first of CERN’s great discoveries came in 1958. It put CERN on the map and was
done with the participation of Maria and Giuseppe Fidecaro, still going strong, who we have
the pleasure of having with us to-day. It produced conclusive evidence that about one
negative pion in ten thousand decayed into an electron and a neutrino as predicted by
weak interaction theory and it put an end to a dispute at the time. As in many SC
experiments in the coming decade all the 6 participants are highly distinguished physicists
and one notes (sir) Alec Merrison and the Nobel prize winner Helmut Paul.
Maria and Giuseppe Fidecaro with experiment 1963
This was the beginning of a golden era of particle physics at the SC which lasted into the
mid-1960’s.
In 1960 the main CERN accelerator at 25 GeV became operational. One would have
expected that the particle physicist would have abandoned the SC and flocked there. This
was not the case. The break through in the theory of weak interactions at lower energies
and parity violation focused attention on a number of key tests using low-energy muons
and pions which were available as ripe fruits for the picking at the SC. A number of the best
and brightest jumped at the occasion with many important firsts such as the pion betadecay, and the muon anomalous magnetic moment. The competition for machine time was
intense and allocutions were in hours, not in number of shifts. The list of participants in
these modest size experiments is an impressive reading and one finds for instance on the
publications names such as G. Charpak, L. Dick, M. Fidecaro, C. Rubbia (very young), V.
Soergel, V. Telegdi, A. Zichichi, K. Winter as well those of a couple of young brilliant
theoreticians like N. Cabibbo and A. de Rujula. Still it was not difficult to perceive that this
bonanza would rather soon run out and that a decision on the future of the SC was urgent.
Here the role of Viki Weisskopf who was the director general 1961-65 was decisive.
He was a visionary with background including nuclear physics and shaped the traditions of
doing physics at CERN in a major way with a series of lasting initiatives. Among his visions
was that particle physics was exposed to a major peril: to succeed brilliantly in its own field
at the prize of isolation from other sciences and that this would lead to its ruin. He saw as
part of his mission to reestablish the traditional links between particle physics and nuclear
physics, which had become tenuous.
Viki Weisskopf
Together with his close friend Amos de-Shalit, then head of the Weizmann Institute,
they had informed a number of people potentially interested that there was to be a meeting
on the interface of high energy physics and nuclear physics at CERN in February 1963.
Since both were very busy, nothing had been organized and questions became persistent as
time approached in the fall of 1962. Weisskopf then asked me to get it going and gave me a
completely free hand. This was an unexpected opportunity for a postdoc.
It is at this point I personally got involved with the isotope separator proposal. I had
come to CERN in 1960 as a postdoc on the Fellowship program from Weisskopf’s group at
MIT and with nuclear physics training in Copenhagen at NORDITA with Ben Mottelson and
Aage Bohr. My nuclear physics profile in the theory group was distinctly different from
that of the other members.
The hastily organized program of the CERN 1963 International Conference on High
Energy Physics and Nuclear Structure (Feb. 25-Mar. 1, 1963) had only 8 one-hour keynote
talks as the only official part of the one-week meeting. The lively informal discussions filled
the rest of the time. It turned out to be a watershed meeting although it had only about 100
participants. It has later continued to this day as the PANIC conferences every three years.
The topics, which concerned the SC, were the following.
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Interactions of pions with complex nuclei
Fragmentation induced by high energy particles
Muons and nuclear structure
Complex nuclei as a tool in elementary particle physics
T. E. O. Ericson
N. A. Perfilov
J. C Sens
V. Telegdi
The Director-General Viki Weisskopf closed the meeting with a call for proposals in
nuclear physics at CERN.
In the absence of a suitable keynote speaker for pions I took it on myself to give the
talk, though with hesitation although I was no expert. Although not an expert and armed
with this I took it on myself The field fascinated me and it led us to a highly fruitful program
pion-nuclear physics for many years in close contact with experiments.
I just happened to be in the right place at the right time.
This meeting was extremely influential and its program largely defined for several
decades what was later called intermediate energy nuclear physics, the central program at
the meson-factories. It led to highly successful experimental program in parallel with the
development of ISOLDE and I will only touch on that and take one example.
An area that paid off handsomely was that of mesonic X-rays, of which the muonic Xrays at nearly the same time were presented as a major argument for meson factories so as
to determine nuclear radii.
Here a major technical advance happened by the not yet commercial introduction of
GeLi crystals which gave about 2 magnitudes better resolution than the previously used
NaI crystals and that changed everything radically. Several groups at the SC quickly
exploited this with locally produced crystals and were in intense competition in particular
with Mme Wu and James Rainwater at Columbia. The CERN groups, which were dominated
in the early stages by Hans Sens and Gerhard Backenstoss, took the upper hand and rapidly
investigated a large part of the periodic system for both muonic and pionic atoms. In
particular the Sens group showed with muonic atoms the explicit effects of nuclear
deformations with a hyperfine effect even for 0 spin nuclei. When the meson factories came
on line, this part of their planned program was mainly superseded.
Time does not permit to go into details of the successes at the SC, but many features
of nuclear studies with pions and nuclei were qualitatively elucidated at the SC, and the
later meson explorations in depth profited greatly from this.
This program however did not set the stage for the on-line separator (later ISOLDE),
which developed in parallel, but it was a logical part of the Weisskopf vision and followed a
different road. After all CERN SC was best accelerator in Europe for an on-line isotope
separator and it had in addition an exceptional environment.
One finds a first trace of this in an internal report of Nuclear Physics (NP) division by
G. Andersson and G. Rudstam concerning an on-line separator dated November 19th 1963.
This was not widely discussed as one sees from the notes on a discussion with Viki
Weisskopf, Pappas, Rudstam and myself in early February 1964 about possibilities of
nuclear structure research at CERN. It mentions nearly as an afterthought radio-isotopes at
the SC and that by special techniques these can be investigated with half-lives down to a
second. The main arguments listed are Q-values and spectroscopy but no perspective is
given. The special technique refers obviously to the on-line separator in view of the
presence of Pappas and Rudstam and in that context one should keep in mind that the
proposal sharply differs from experimental proposals. It concerned a program, not an
experiment, and it was an enterprise of a size well beyond the resources of the nuclear
chemistry group.
I was not in contact with the developments of the project, but by the end of August
the same year there is a memorandum that Peter Preiswerk, the Nuclear Physics Division
Leader, and I sent to Weisskopf proposing the creation of a special nuclear structure
committee since the usual committees for experiments lacked expertise in the field. I think
this initiative was due primarily to Preiswerk with the separator proposal in mind.
Anyway, 2 month later one morning Preiswerk walked into my office and announced that I
now was the chairman of this newly created committee so far without members and that I
should present the isotope separator project to the Nuclear Physics Research Committee
the next morning. During a sleepless night I was impressed. Assuming that it worked
technically it was obvious that it opened a host of possibilities for nuclear physics. That is
how it came that I enthusiastically recommended the physics of the project to that
committee although I knew nothing about the practical details.
This is how I became an instant expert and how my opinion showed to non-specialists
that independent nuclear expertise had been consulted and supported the project.
Wolfgang Gentner
Peter Preiswerk 1907-72
In the notes on the preparations for a final proposal from November 19th 1964 there
is a detail in the minutes that give an interesting insight into the life of physicists at the
time. A discussion concerns the need for a computer for on-line work and the memory size
is set at about 1 kilobyte. The price tag at the time for this modest performance is $24000, a
very large sum corresponding to about 30 times more to-day.
To-days computers were still far away.
With the encouragement by NPRC a polished final version of the project was now
submitted and approved on December 17th.
It is interesting to ask who were the hidden and crucial supporters. In my opinion it
was no accident that Gentner, Pappas and Preiswerk all had a nuclear physics background
linked to the Joliot-Curie group in Paris and they all were had great influence at CERN in
various ways. They would all have been well aware of the possibilities of such an isotope
separator and the strong positive impact of the CERN infrastructure. If these crucial people
and Weisskopf had come with a particle background, ISOLDE might never have happened.
Now the ISOLDE program is no longer fragile and exposed to timely coincidences. It is
an integral part of CERN and will presumably last as long as CERN exists.
GOOD LUCK ISOLDE FOR MANY FRUITFUL YEARS TO COME!
Who would have guessed my wish to-day in 1964!