Introduction
Erich Regener was born in West Prussia in 1881. He studied physics at the
University of Berlin, and his 1905 thesis (Regener, 1905) concerns the
interaction of UV light with oxygen and ozone molecules and the equilibrium
between them, a topic that he revisited on several occasions. In Berlin he
chose as the subject for his Habilitationsschrift the determination
of the elementary electric charge using different methods, including
alpha-particle scintillations (Regener, 1911). His accurate determination of
e is very close to today's accepted value.
After a period as professor at the Agricultural University of Berlin and as
X-ray field technician during the 1914–1918 war, Regener became a full
professor of physics at the Technical University of Stuttgart in 1920. A hot
topic at this time was the study of atmospheric ionisation following the 1912
discovery of cosmic rays by Victor Hess (1883–1964), and, from the second half of the 1920s,
Erich Regener made significant contributions to the study of this field, both
through work that he carried out himself and through stimulating work carried
out by his son and by his students, both with him and independently: some of
these activities are related in Appendix A. Regener realised that ionisation
measurements had to be made deep in lakes and at the highest altitudes and
developed the necessary instruments, along with innovative recording
techniques, to make such measurements.
Because his first wife, Viktoria, was a Russian-born Jew Regener had to take
“provisional retirement” in 1937 for refusing to divorce her (B. Hoerlin,
2011). While his son and daughter emigrated, Regener started a private
research institute on Lake Constance in 1937 that was supported by the Kaiser
Wilhelm Society and soon became part of the Society. He was recruited by
Wernher von Braun to design instruments for rocket flights that were initially
to be used to measure the temperature, pressure and density in the upper
atmosphere. After World War II Regener became the first vice president of
the Max Planck Gesellschaft (formerly the Kaiser Wilhelm Gesellschaft) and was
reinstated to the chair in Stuttgart, which he held until his retirement in
1951. In 1952 his private institute was incorporated into the,
Max-Planck-Gesellschaft as the Max-Planck-Institut für
Sonnensystemforschung. Regener died in 1955: an obituary written by the Nobel
Laureate P. M. S. Blackett appeared in Nature (Blackett, 1959).
Cosmic ray work at Stuttgart
Regener began his cosmic ray work in Stuttgart in 1928. In that year he
noted (Regener, 1928) that the study of the “Hess radiation” is difficult
for several reasons: the instrument can contain radioactive material, the
air can contain radioactive emanations and the ground contains radioactive
radium. Any instrumental radioactivity can be investigated by measuring the
ionisation in deep lakes, while measurements at high altitudes avoid the
ground radioactivity.
Regener was an ingenious experimentalist and soon established a depth record
for observations of the rate of production of ion pairs. He tackled the
problem of measuring the decrease in ionisation as a function of increasing
depth by submerging a quartz-fibre electrometer and a pressure chamber in
the waters of Lake Constance. He succeeded (Regener, 1933a) in measuring the rate
of ionisation (pairs of ions produced cm-3 s-1) down to a depth of
231 m, where the rate was found to be very low and of the order 0.05 ion pairs cm-3 s-1.
Millikan and Cameron (1928a), working in the US, reached a
depth of 50 m, where the ionisation was about 2.6 ion pairs cm-3 s-1.
The “steel bomb” in which Regener measured the ionisation weighed 130 kg and
contained CO2 at a pressure of 30 atmospheres. His early work, carried
out using a hand-operated winch on a rowing boat, was so successful that it
attracted sufficient funding to purchase a motor boat for later work. He
named this boat “Undala”, reflecting his belief, and one that was widely held
at that time, that cosmic rays were photons with wave-like properties rather
than particles. Interesting accounts of the underwater research have been
given by Paetzold et al. (1974) and by Pfotzer (1985). Pfotzer worked with
Regener, first as a student from 1929 and later, after the war, as a senior
scientist.
Regener's work in Lake Constance (Regener, 1931) was highly regarded by
eminent physicists of the day, including Bruno Rossi (1905–1993), a close
friend despite their differing views on the nature of cosmic radiation
(Rossi, 1985). At the Royal Society discussion meeting of 1931 (Geiger et
al., 1931), attended by the likes of Geiger, Rutherford, Wilson and
Bragg, both Geiger and Rutherford praised Regener's work although they
disagreed as to whether it provided evidence for the particle (Geiger) or
wave (Rutherford) nature of the radiation. In Rossi's 1985 article there is
a photograph of Rossi with Lise Meitner – also a supporter of the wave
hypothesis – and Erich Regener (two of Rossi's “dearest and most respected
friends”) aboard Regener's boat on Lake Constance. Part of the caption reads
“Regener had baptized the boat “Undula” to reaffirm his faith in the wave
nature of cosmic rays. Here, following a discussion on this subject, he was
telling me: “If it turns out that you are right, I would have to rename my
boat “Korpuskel”, which does not sound as nice as “Undula”.”” No date is
given for this photograph, but it was presumably taken in the early 1930s and
before the discovery of the East–West effect that did much to establish the
particulate nature of the radiation (Johnson, 1933; Alvarez and Compton,
1933). A photograph of Regener working aboard the Undula is shown in Fig. 1.
In the early 1930s Regener started a research program aimed at extending the
atmospheric measurements of Hess and of Werner Kolhörster (1881–1945),
who, in 1914, undertook a flight to 9300 m. Millikan and Bowen (1926) had found it
difficult to keep their instruments working at the cold temperatures
encountered at high altitudes, and Regener overcame this problem by enclosing
his equipment in a light gondola encased in cellophane. The cellophane
turned the gondola into an effective greenhouse against the low temperatures, and the apparatus was operated at what was effectively room temperature.
Again, Georg Pfotzer (1909–1981) has given a good description of the
challenges Regener faced and the solutions that he adopted (Pfotzer, 1985).
Erich Regener, on the left, on board the Undula on
Lake Constance ∼1933.
Credit: Archiv der Max-Planck-Gesellschaft, Berlin-Dahlem.
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Results with a balloon-borne electrometer and the<?xmltex \hack{\\}?> energy density
of cosmic rays
Between 1932 and 1934 Regener undertook several flights during which he succeeded
in making measurements of the rate of ionisation up to heights in the
atmosphere where the overburden was less than 50 mm Hg
(∼50 g cm-2). The data from the first flight are shown in Fig. 2.
The maximum height reached was determined by the point at which one of the
three or four lifting balloons burst, ozone interacting with the rubber
probably being the cause of the rupture.
Ionisation data from Regener (1932) taken using an
electrometer and a 2.1 L ionisation chamber. The ionisation rate
(ion pairs cm-3 s-1) is shown as function of decreasing pressure
(mm Hg). Also shown are data from Kolhörster and Piccard. The extrapolated
value is shown as 275 ion pairs cm-3 s-1. As was customary at the
time, no errors are shown on the data points. Using these data Regener was
able to confirm the results of Kolhörster and infer that the rate of
production of ionisation had reached a maximum value, which he extrapolated to
275 ion pairs cm-3 s-1. He defined this as the “intensity of the
cosmic radiation” at “its entrance to the atmosphere”.
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In a paper submitted on 31 December 1932 (Regener, 1933b) and published only
4 weeks later, Regener reported the results of integrating the ionisation as
a function of height to obtain the total number of ions, produced by the
absorption of cosmic rays by a column of air of 1 cm2 cross section, as
1.02 × 108 ion pairs, to be compared with the value of
1.28 × 107ion pairs estimated by Millikan and Cameron
(1928b) from a larger extrapolation. Taking 32 eV (about 6% lower than
modern measurements) as the energy needed to produce an electron–ion pair in
air, he deduced that the energy reaching the earth in the form of cosmic rays
was 5.2 × 10-2 erg cm-2 s-1. In a later paper
(Millikan et al., 1933), a value of
3.2 × 10-2 erg cm-2 s-1 was derived and compared
with the energy falling to the earth as starlight,
6.91 × 10-3 erg cm-2 s-1. Millikan and his
co-authors made no reference to Regener's paper although it had been
published, in English, nearly 6 months prior to their own work being
submitted.
Of course, both values for the energy flux are low as both Millikan and
Regener were unaware that the bulk of the cosmic rays are charged particles.
Both subscribed to the photon view, and the values are about a factor 2 lower than modern estimates. These numbers were used by Baade and Zwicky in
their classic paper as evidence to support their theory that cosmic rays
were produced in supernovae (Baade and Zwicky, 1934).
Ionisation data from Regener (1933c). The ionisation rate
(ion pairs cm-3 s-1) is shown as function of decreasing pressure
(mm Hg). Results from four flights during 1932–1933 with data corrected to
one atmosphere pressure. Also shown are Kolhörster's data.
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From a series of balloon flights in 1932 and 1933, Regener reported the data
shown in Fig. 3. The three lower curves of Fig. 3 are in good agreement, and there is evidence that the rate of production of ion pairs has reached a
maximum value. It was normal practice to correct the rate of production of
ion pairs to a pressure of one atmosphere: the pressures in the ionisation
chamber monitored by the electrometer varied considerably over the range of
three to four atmospheres in different flights.
The results from the flight of 29 March 1933 are of particular interest. The
instrument was apparently working well, and no instrumental reason could be
identified to account for the much higher rate of ionisation. Regener
considered the possibility that radioactivity from the moon might be
responsible. He rejected this as he did the possibility that “a magnetic
disturbance of medium strength” that occurred on that day, while it was
magnetically calm on the other days, had an influence. He noted that “it
would be remarkable if there were a connexion between the magnetic intensity
and the intensity of cosmic rays in the highest part of the atmosphere, and
only in the highest parts: that is to say, that there are additional rays
(soft rays) there perhaps coming from a sunspot”. Of course, with the
benefit of hindsight one might remark that the solar connection should have
perhaps been given more attention, but the statement demonstrates
the strong adherence that Regener, and others, had to the photon hypothesis.
Indeed he discusses the shape of the curve at the highest altitudes as being
evidence of an electromagnetic component.
Discovery of the maximum
In his 1933 Nature paper (Regener, 1933c), Regener noted that the ionisation
data is for rays coming from all directions. He then went on to report that
his collaborator, Bernhard Gross, had calculated the ionisation data for
vertically incident rays, showing that the intensity diminished towards the
top of the atmosphere, i.e. shows a maximum. In that paper (Gross, 1933), received on 6
April 1933 and published 14 June 1933, Gross acknowledges Regener for
suggesting the problem and for his help with the work. The results of
Gross's calculations are shown in Fig. 4.
The ionisation rate (ion pairs cm-3s-1) as
function of decreasing pressure (mm Hg) from the work of Gross (1933). Line I
shows the measured rate (Regener data) for rays from all directions. Line II
is the calculated rate for vertically incident rays, showing a clear maximum
at about 130 mm Hg and a shoulder at 350 mm Hg.
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As an excellent experimentalist, Regener wanted to exploit the
Geiger–Müller (GM) tubes, developed in 1928, so that he could register single
charged particles. He involved Georg Pfotzer, one of his students, in this
work. Their first results, obtained with one tube carried to an altitude of
28 km, were published in September 1934 (Regener and Pfotzer, 1934), with the
ionisation measured found to be the same above 18 km height as reported
earlier with an ionisation chamber.
A year later, in November 1935, Regener and Pfotzer described the results
from two ascents with three GM tubes in coincidence, arranged to measure the
vertical intensity. The solid angle was 20 degrees about the zenith (Regener
and Pfotzer, 1935). Data show a clear maximum for 100 mm Hg pressure
(∼16 km) and a bump at 300 mm Hg; see Fig. 5. Note the
author ordering in these two Nature papers.
Data from coincidence measurements by Regener and Pfotzer
(1935). The coincidence rate per 4 min at a solid angle of 20 degrees
about the zenith is shown as function of decreasing pressure (mm Hg) (data
points and line I). Data, corrected for dead-time losses, are shown as line
II. Note the clear maximum at about 100 mm Hg.
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Pfotzer gives a detailed account of this work together with results from
other flights in his 1936 thesis, presented on 15 May 1936 and in part
published in Zeitschrift f. Physik (Pfotzer, 1936). In the acknowledgements
Pfotzer thanks Regener warmly: “My sincere thanks to my idolized teacher
Prof. Regener for organizing the ascents and for continuous support and
help.”
Pfotzer went on to have a distinguished career in physics, following Regener
as Director of the Max-Planck-Institut für Stratosphärenforschung
in 1956. However, it is clear to us that the real discoverer of the maximum
in the rate of production of ion pairs with altitude, misleadingly called
the Pfotzer maximum, was Erich Regener, with the 1935 result essentially
confirming those shown in Fig. 4.
Why not the Regener maximum or even the<?xmltex \hack{\\}?> Regener–Pfotzer
maximum?
It is not at all clear why Regener's name is not now associated with the
maximum in the altitude dependence of the rate of production of ion pairs. In
their classic paper (Bhabha and Heitler, 1937), submitted in December 1936,
Bhabha (1909–1966) and Heitler (1904–1981) use the work of Regener and
Pfotzer, along with the Rossi transition curves (Rossi, 1938), as
demonstrations of the accuracy of their theory of cosmic showers. Phrases
such as “Regener's measurements of the absorption of cosmic rays in the
atmosphere” are used, and in the discussion of comparisons of theory with
experimental data (Sect. 7B), the title of the section is “The Regener Curve
at High Altitude”. Although Pfotzer's 1936 paper is in the reference list
and, curiously, the order of the authors in the citation of Regener and
Pfotzer (1935) has Pfotzer's name first, it is clear that Regener is seen by
no less authorities than Bhabha and Heitler as the discoverer of the maximum.
Heitler must surely have known what was going on in Germany, and certainly
Bhabha, at that time working in Cambridge in Rutherford's laboratory, will
have been aware of the close links of Rutherford and Blackett to Regener
(Carmichael, 1985). In a paper written in 1938, Rossi (1938) speaks of
the Regener–Pfotzer curve at 100 g cm-2: no citation is given.
However, in 1941, Schein, Jess and Wollan (Schein et al., 1941) showed a plot of the
ionisation as a function of altitude in which the maximum is labelled “the Pfotzer maximum”. In this paper, and many subsequent papers,
the maximum is generally and, we contend, misleadingly referred to as the
Pfotzer maximum, without citation.
It is our speculation that Regener advised Pfotzer against joint authorship
as, by 1936, his difficulties with the National Socialist Party were
increasing. A fascinating account of these difficulties, based on letters
between her parents, has been given by the daughter of one of Regener's
students (B. Hoerlin, 2011). In 1933, during the period of book burning in
Germany, Regener, along with the Jewish librarian in Stuttgart, were rumoured
to be targets of angry students supportive of the National Socialists.
Accordingly, Hermann Hoerlin appointed himself as guardian of Regener and his
family and, armed with a wooden ice axe, slept on the floor behind the door
of Regener's apartment. Bettina Hoerlin also notes that Regener was one of 75
established physicists (including Heisenberg) who, in October 1936, sent a
petition to the Reich's Minister of Education warning of the declining state
of German physics. She remarks that “the contrarian act of signing the
petition” – along with the issue of his wife – “sealed Regener's removal
from the Stuttgart Faculty”.
Regener was nominated by Schrödinger for the Nobel Prize in Physics in
1938 for his detailed measurements of ionisation rates, both in the
atmosphere and deep under water.
It is our firm conclusion that the atmospheric and cosmic ray communities
should start talking and writing about the Regener–Pfotzer maximum, or even
simply the Regener maximum, when discussing the position in the atmosphere at
which the rate of production of ionisation becomes a maximum.