Heavy and ultraheavy nuclei observations of cosmic rays using ionization chamber-Cerenkov counter syste

Binns, W. R.

Fernandez, J. I.

Israel, M. H.

Klarmann, J.

Mewaldt, R. A.

English

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", `- 1 
 i: F ^7'-'v -z
OCT 1971
Laboratory for
Space Physics
Department of Physics
Washington University/St. Louis
6
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C14 N.2 • 3 ^s Y
(ACCL S I ON BE R)
5.
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(NASA CR OR TMX OR AD NUMBER)
(CODE
(CAUGOWY)
-J
OBSERVATION of VH and VVH COSMIC RAYS
WITH AN
IONIZATION-CERENKOV DETECTOR SYSTEM
J. I. Fernandez, { M. H. Israel, + J. Klarmann,+
R. A. Mewaldt, + and W. R. Binns
Paper OG -
12th international Conferer	 on Cosmic Rays
Hobart, Tasmania Aug. 1971
*McDonnell Douglas Research Laboratories, St. Louis, Mo. 63166
+Departm ent of Physics, Washington University, ;;t. Louis, .o. 63130
OBSERVATION OF VH AND VVH COSMIC RAYS
WITH AN IONIZATION-CERENKOV DETECTOR SYSTEM
W.R.Binns ,+ J.I.Fernandez ,* M.H.Israel ,* J.Klarmann ,* R.A.Mewaldt*
*Dept. of Physics, Washington Univer Ity, St.Louis,Mo. 63130 USA
+McDonnell Douglas Research Laboratories, St. Louis, Mo. 63166, USA
A detector system including parallel-plate pulse ionizatiIon chambers .37d
a Lucite Cerenkov counter, having geometry factor 0.92 m ster, was opera-
ted during a series of balloon flights near 4.5 g/cm 2
 atmospheric depth
at geomagnetic cutoffs near 4.5 GV and 1.6 GV. Charge spectra and flux
ratios are presented. The VH results are consistent with a cosmic-ray
source having J (21 < Z!524)  - 0, and J(20)/J(26)  _ .12 + .06.  The VVH re-
sults are consistent with the dominant mode of nucleosynthesis for these
nuclei being rapid neutron capture (r-process).
-1.
	 IntrodUction. We are reporting preliminary results of a series of high-
Altitude galloon flights to measure the charge spectrum of cosmic rays in the
Very-Heavy (VII) region, i.e., nuclei of charge (Z) 20 to 30, and in the Very-
Very-Heavy (VVH) region, i.e., nuclei of Z---30, our detector system identifies
the charge of the particles by measuring energy-loss in parallel-plate pulse
ionization chambers and measuring intensity of Cerenkov light emitted in a sheet
of Lucite. The detectors have active area nearly 1 m 2 , and the geometry factor
of the system is 0.32 m 2ster. The instrument was flown frozn the couth urn USA
near geomagnetic cutoff 4.5 GV in three flights in January and May, 1970. It
was also flown in September 1970 from Sioux Falls, S.D., where the geomagnetic
cutoff is below the Cerenkov counter threshold. In all flights the balloon
float altitude was near 4.5 g/cm2 . Float data was recorded -'or 28 hours near
4.5 GV and for 18 hours in the flight at low cutoff.
2.	 Detector system.
Figure 1 shows the
cross-section of the de-
tector used in the first
three flights. The prin-
cipal components of the
system are the two par-
allel-plate pulse ioni-
zation counters and the
Lucite Cerenkov counter.
The pulse height from
each of these three de-
tectors is recorded for
each heavily charged
particle that pene-
trates the system. The
trajectory of the par-
'	 `--UPPER HODOSCOPE
IONIZATION CHAMBERS
GUARD COUNTERS
CERENKOV
PHOTOMJLTIPLIER TUBES
ADIABATIC LIGHT PIPES
LUCITE CERENKOV RADIATOR
LOWER HODOSCOPE
^------METER	 HODOSCOPE
PHOTOMULTIPLIER TUBES
Fig. 1
ticle is given by a hodoscope consisting of two crossed layers of scintillator
strips above the other counters and two crossed layers below. The hodoscope
1:
	 also serves to define the acceptance cone of the system.
The ionization chamber plates are separated by 5 cm of gas, 90% Argon and
1.0% Methane at one atmosphere. Ionization chambers are used for the energy-loss
I-p;.f(0)
Co kz'(1-A—in,)
.	 ^I • g (pl
Snn
measurement because they are easily constructed in the large areas required
and they Oo not show the non-linearity at large energy-loss which is charac-
teristic of plastic scintillators. These chambers have been described more
fully by Epstein et al (1971). The Cerenkov counter is a sheet of ultra-
violet-transmitting Lucite 0.63 cm th-ck viewed on the edges by four 12 cm di-
ameter photomultiplier3 through adiabatic light pipes. For the lass, flight of
the series, the one at low cutoff, this total internal reflection counter
was replaced by one using diffuse reflection for more uniform light collection.
This latter counter ha. the Lucite radiator placed inside a white box, viewed
by eight photomultipliers around the sides.
F4 gure 2 illus-
trates the familiar 1C
response of such an
energy-loss-Cerenkov
detector system.	 Both
energy-loss and	 Cer-
enkov radiation vary
di-rectly with the
square of the nuclear p
charge, but their de-
DXpenden;;e upon the energy
of the particle is dif-
ferent.	 Thus par-
ticles of different
charge and energy	 fall 2
along a series of curve:.
This plot illustrates
two problems of inter-
pretation of our data.
First, the rel.a—	 CERENKOV PULSE HEJGNT
tivistic rise of ener-
gy -loss results in a
	 Fig. 2
r(-.-ion of ambiguity where,
for example, an iron nu-
cleus (Z=26) with energy near 10 GeV/nucleon gives the same energy-loss and
Cerenkov-light as a nickel (Z=28) nucleus near 2 GeV/nucleon. Because of the
steepness of the cosmic ray energy spectrum, this ambiguity seriously affects
the charge spectrum only where the lower charged particle is very much more
abundant than the higher charged particle. Since the cosmic-ray charge spec-
trum has a. large peak at iron and very low abundances at higher charge, the re-
sults we present for cosmic rays of charge 28 through 31 will be confined to
the lower energies outside t..is region of ambiguity.
The second source of ambiguity in interpretation of our results is pecul-
iar to our detector and is significant for measurement of the lower energy nu-
clei with charge below 26. Incident nuclei which penetrate the ionization
chambers and then suffer a nuclear interaction somewhere inside the Lucite
Cerenkov radiator will register an improperly low Cerenkov signal. Thus, for
example, an iron nucleus of 2 GeV/nucleon interacting in the Cerenkov counter
will be displaced to the left in this plot and may be incorrectly identified as
a 600 MeV/nucleon particle of charge 23. The Lucite thickness is 7% of an inter-
2
14 16 18 20 22 24 26 28 30 32 . 34 36
CHARGE ---w
Fig. 3
action length for iron and the low energy nuclei whose measurement is affected
are much less abundant than the relativistic iron. As a result our observa-
tions of nuclei with charge below 26 and energy below about 1 GeV/nucleon must
be taken as upper limits to the true flux. To eliminate this source of ambi-
buity in future flights, a third ionization chamber, below the Cerenkov coun-
ter, has been added to our system. Requiring agreement between the energy-loss
in the chambers above the counter and the energy-loss
 in the chamber below the
counter will eliminate most events in which a particle interacts in the Ceren-
kov counter.
3.	 Results.
Figure 3 is
a charge histo-
gram for 16,000	 u,1
events observed	 Z
near 4.5 GV cut-	 D
off. The charge
	 0
is determined by
taking the mean
of the two ioni-
zation chamber
signals and cor-
recting for the
velocity of the
particle as de-
teimned with
the Cerenkov
counter. For
particles above
4.5 GV , 1-1e ion-
ization is very
nearly constant
with energy and
the Cerenkov correction has only a small effect on the charge resolution, al-
tbough it is extremely useful in verifying that the particles are actually rela-
tivistic. In addition to the prominent peak at iron, we also observe peaks --t
charge 14, 16, 18, and 20. The peaks at 14 and 16 are distorted by theioniza-
tion chamber discriminatc•r and so are not representative of the true flux of
these nuclei. The charges between calcium and iron are not resolved but their
fluxes are low compared to iron. For example, the flux of chromium (charge 24)
is not more than 25% that of iron. In general, our results in the VH region
agree with other recent counter measurements (Leznia k et al, 1969; Dayton,
et al, 1969). We have better statistics but poorer charge resolution.
The normalization of the charge scale is based on the assumption that the
center of the prominent peak is exactly at charge 26. However, we have an in-
dependent verification that this peak is in fact iron. Each ionization chamber
has a built-in radioactive source of 5.5 MeV alpha particles. The ionization
chamber signal due to these alpha particles is measured periodically throughout
the flight. Calculations show that the mean signal due to these alphas in our
chamber should have the same pulse height as a vertically incident cosmic-ray
nucleus of 2.5 GeV/nucleon with charge 21.5 + 0.4. (The uncertainty in this
number is a conservative estimate, principally due to uncertainty in the
3
t 00
\200
ZU 100
0
distribution of energy-loss along the path of the stopping alpha-particle.)
Using this in-flight calibration, the peak in the cosmic-ray charge spectrum is
at charge 26.1 + 0.5. The width of the peak (approximately 1.5 charge units,
fwhm) implies that it consists mostly of particles of one nuclear charge. We
therefore have direct evidence that this peak consists of iron nuclei.
The arrows in Figure 3 represent particles we have observed with chargL-s
greater than 30. In the case of these relativistic particles, the charge as-
signment agrees in all three counters, and so we are quite confident that they
are actual VVH particles and not fluctuations from the iron peak.
It is always necessary to extrapolate observed data to the top of the at-
mosphere. With our very large geometrical factor, we can use the data gathered
during the balloon ascent to measure directly the atmospheric attenuation of
these nuclei. We find an attenuation mean free path for iron nuclei of 15.6 +
2.2 g/cm2; and for the VH group, 19.7 +1.8 g/cm2.
400
Figure 4 shows the charge spec-
trum for 6200 particles whose energy
at the top of the atmosphere was 0.6
to 1.3 GeV/nue- l eon, as measured in
the flight at low geomagnetic cutoff.
As mentioned before, the fluxes be-
low iron in this energy range must
be treated as upper limits; however
the fluxes above iron are reliable
because there is no ambiguity due to
relativistic rise of energy loss in
this energy interval. Note the
clump of particles near charge 28
which gives us confidence that we
	 Fig. 4
are really observing particles of this charge and not just a tail on the iron
distribution.
4.	 Discussion.	 The following table summarizes flux ratios for the VH nuclei
above 4.5 GV:
J (21 s Z t624)
	
J (Ca)
J(25S Z <_30)
	
J(Fe)
Observed at detector	 .42 + .04	 .27 + .04
600 M ev/rucleon
-1.3 G evA ucloon
DGMIANAMR
18	 20	 22	 24	 26	 28	 30
CHARGE
Extrapolation to top of the
atmosphere
Source: slab model
steady state model
Solar system (Cameron, 1967)
	
.35 + .06	 .21 + .05
	
assume zero	 .12 + .06
	
assume zero	 .11 + .06
.02	 .08
Calculating with simple models of the galactic propagation, we can ac-
count for all the observed nuclei in the -Uarge 21 through 24 interval as frag-
mentation products. In a slab model this requires a slab thickness of 2 g/cm2.
In a steady state leaky box model a leakage length of about 5 g/cm 2 is required,
consistent with that derived from the ratio of light to medium cosmic rays;
4
however, in this model the losses of VH nuclei are due mainly to interactions,
and the relative fluxes are not very sensitive to the leakage length. In either
of these models we find that about half of the observed calcium has resulted
from fragmentation, implying that the excess calcium was originally present at
the source. The resulting calcium to iron ratio at the source is consistent
with the solar system abundances.
In the energy interval of 0.6 to 0.9 GeV/nucleon the ratio J(21 _<Z-,_,1t24)/
J(25 4 Z !C 30) is observed to be 0.67 + .05. This is an upper limit to the cor-
rect ratio, as previously noted, but it is still consistent with all nuclei in
the 21 through 24 interval being fragments of heavier nuclei because of the
greater probability that an interacting iron will fragment into this group at
these lower energies. (Waddington, 1969, gives the probability of fragmenta-
tion of iron into this charge group as 0.41 for energies greater than 2.1
GeV/nucleon, and 0.60 at 0.7 GeV/nucleon.)
The following table summarizes our results for nuclei heavier than iron.
The results are expressed as a ratio of flux in the given interval to flux in
the "iron peak" (256.Z,<27). For charges 28 through 31 the results are for the
energy interval 0.6 to 1.3 GeV/nucleon. For higher charges the results are for
all events observed with energy greater than 0.6 GeV/nucleon. (2.9 x 10 4 such
particles were observed in the iron peak.)
Z
	
this experiment	 solar system	 emulsions
(Cameron, 1967) 	 & plastics
28 - 29
30 - 31
33
40
*Blanford et al (1969)
**Fowler et al (1969)
.035 + .005
.0015 + .001
(8 + 27 x 10-4
(2 + 1) x 10-4
.05
.002
3 x 10-4
.5 x10-4
1.3 x 10-4
.7 k 10-4 ^*
Our results are in rough agreement with the solar system abundances. The dis-
agreement with the emulsions and plastics above charge 33 may be due to an er-
ror in determination of the charge thieshold for the plastics.
Finally, an important aim of the study of VVH cosmic rays Is to obtain in-
formation on the nature of the cosmic ray sources. Supernova explosions are
likely candidates and it is believed that nucleosvnthesis by the rapid neutron
capture process (r-process) occurs in such explosions. In the charge interval
between 34 and 40, the charge spectrum expected from an r-process is quite dif-
ferent from that of slow neutron capture, (the s-process). We nave calculated
the expected value of the ratio J(34,<  Z <,36)/J(37,< Z -^ 40) in cosmic rays using
galactic propagation in a steady-state leaky box model and fragmentation cross-
sections from Shapiro & Silberberg (1970). Taking r-process source abundances
(Seeger et al, 1965), we find a ratio of 7 or 8; with s-process source, the
ratio is between 1.1 and 1.4. We observe the ratio 11/2=5.5. Within its rather
poor statistics, this observation favors the r-process over the s -process.
Acknowledgements. This work was supported in part by NASA under grant NGR
26-008-001 and in part by McDonnell Douglas corporat4on, Independent Research
and Development Program. One of us (MHI) is supported in part by the Alfred
P. Sloan Foundation as a Sloan Research Fellow.
5
I-
References.
Blanford, et al, 1969, Phys. Rev. Letters 23, 338.
Cameron, A.G.W., 1967, Origin and Distribution of the Elements (Pergamon Press,
1968) 125.
Dayton, B, N. Lund, & T. Risbo, 1969, Acta Physica Hungaricae 29, suppl 1, 375.
Epstein, J.W., J.I.Fernandez, H.H.Israel, J.Klarmann, & R.A.11 "Iewaldt, 1971,
Nuclear Inrtruments & Methoda, in press.
Fowler, P.H., J.M.Kidd, R.T.Moses, 1969, Acta Physica Hungaricae 29 Suppl 1,399.
Lezniak, J.A., T.T. vonRosenvinge, & W.R.Webber, 1969, Acta Physica
Hungaricae 29 Suppl 1, 375.
Waddington, C.J., 1969, Astrophysics and Space Science 5, 3.
t	 Seeger. P.A., W.F.Fowler, & L.D.Clayton, 1955, Astrophys. J. Suppl. 11, 121.
Shapiro, M.M., & R.Silberberg, 1970, Ann. Rev. Nucl. Sci, 20, 323.
6


Observation of VH and VVH cosmic rays with an ionization-Cerenkov detector system

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