Properties of ionization spectrometer exposed to 10, 20.5, and 28 GeV/c synchrotron accelerated proton

Huggett, R. W.

Jones, W. V.

Pinkau, K.

Pollvogt, U.

Schmidt, W. K. H.

English

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Paper. No. 140
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Properties of an Ionization Spectrometer Exposed to
10, 20.5, and 28 GeV/c Machine Accelerated Protons.
W.V. Jones, K. Pinkau, U. Pollvogt, W.K.H. SchmtTU_
Max-Planck-Institut fur Physik and Astrophysik
Institut fur Extraterrestrische Physik
80+6 Garching bei Munchen, Germany
and
1
R.W. Huggett
Department of Physics and Astronomy
"Zouisiana State University /y G_ 'f-/9- a 0 0 2—
BatonRouge, Louisiana 70803, U S A
ArQrrOnr .
An ionization spectrometer has been employed in several
successful balloon flights in combination with spark
2hambers and different target materials to study high
anergy cosmic rays. The same spectrometer was exposed
to 10, 20.5, and 28 GeV/c protons from the Alternating
Jradient Synchrotron (AGS) at Brookhaven, in order to
study its response and be able to calibrate it at these
energies. Some results of these calibration measurements
are reported here. These include the accuracy of energy
measurement, the inelastic interaction cross section for
protons in iron, and the inelasticity of the interac-
tions of protons with carbon, iron , and lead nuclei.
FResearch supported ,jointly by the National Science
Foundation under Grant No. GP/716 a^nd by Deutsche
Forschungsgemeinschaft under Grant No. Pi 34/5.
- 2 -
I. Introduction
Ionization spectrometers of the type described in this
paper have been in use to determine the energy oz ultra
high energy particles since 19571).
The ionization spectrometer discussed here had a depth
of only three interaction lengths, because the weight
that could be carried by balloons appeared to be limited
at the time of conception of the experiment. Theoretical
calculations showed that an ionization spectrometer of
limited depth should be still useful for energy deter-
mination, since it appeared that, on average, the maximum
of the cascade development would occur within a depth of
three interaction lengths2).
It was expected that basic knowledge on the behaviour of
the cascade development in the spectrometer could be
obtained by exposing the apparatus to particles of known
energy from an accelerator. This report is devoted to a
description of the studies that have been carried out at
the Brookhaven Alternating Gradient Synchrotron (AGS).
II. Experimental Procedure
A brief description of the apparatus has already been
given in Ref. 3). The ionization spectrometer (see Fig.1)
consisted of 6 slabs of iron, each of depth 7 cm (i.e.
55 g/cm2 ), and a cross sectional area of 18 x 18 cm 2. In
between these slabs and below the lowest there were 1 cm
thick plastic scintillators inserted to sample the ion-
ization loss of the cascades. Above the spectrometer
there were two spark chambers of 4 and 12 gaps, respec-
tively, and in between them a target space of dimensions
_3_
18 x 18 x 18 cm3 . Above the upper spark chamber and below
the target there were two beam defining scintillators of
cross sectional areas of 15 x 15 em2.
For triggering purposes each scintillator was viewed by
one photomultiplier. The outputs of the PM tubes were
connected to various threshold discriminators. If a pulse
exceeded a threshold, a light bulb was turned on and photo-
graphed by two Bolex 16 mm movie cameras along with the
spark chambers and the display of three multi-channel
analyzers described below. The threshold discriminators
of counter T8 were set to show more than 4 times, 9 times,
and 16 times minimum ionization (corrected for non linear
light output of plastic scintillators for heavily ionizing
particles). In T7 the passage of 2 or more minimum ionizing
particles was registered. In counters T1 to T6 the passage
of two or more and 13 or more particles was registered.
Counter-0 was used in anticoincidence to exclude side show-
ers from being registered. For measuring ionization loss,
three photomultipliers (MI to MIII) were employed which
looked at a pair of scintillators each. The output pulses
of these tubes were analysed in three 128 channel analy-
sers of logarithmic response.
During balloon flights the apparatus was triggered when-
ever counters T7 and T8 registered at least 1 times mini-
mum ionization and at the same time each scintillator of
either one of the pairs A, B, or C registered at least 13
times minimum ionization. Thus the coincidence requirement
for balloon flights was (A + B + C) T7 T8 "6 = 1.
For triggering during the AGS exposure,only one particle
was required to traverse T7 T8 , and 2 more beam
defining counters in front of the apparatus. Because of
the registry of pulses exceeding thresholds as mentioned
- 4 -
above, from all AGS events,those events could be selected
which would have triggered the apparatus under balloon
flight conditions. Thus it was possible to derive the
change of triggering efficiency of the apparatus with
incident energy.
During the AGS exposure there was on average one proton
incident on the apparatus during each AGS pulse. Multiple
incidence could be recognized by scanning the spark cham-
ber pictures. The spark chambers had a very good multiple
track efficiency. The number of events where two protons
followed each other closely in time while only one of them
was registered by the spark chambers and the other one
possibly distorted the pulse height measurement was found
to be negligible, mainly due to linear gates which were
sensitive for less than 0.5 rsec for the pulses to be
measured after the apparatus had been triggered.
Three different energies of incident protons were selected:
100 20.5, and 28 GeV. At each energy about 5,000 events
were recorded for each of two different targets in the pro-
vided space: a lead target of 86,5 g/cm2
 and a carbon tar-
get of 33,5 g/cm . The same targets had been used in
balloon flights before. In addition,at 28 GeV,a data set
was recorded with the target space empty.
III. Results
For the results described in this and the following sec-
tions,only those events were used it which the primary
proton was incident cn a cross sectional area of 8 x 8 cm2
about the center of the cross sectional area of the iron
absorber, i.e. the cascade axes were at least 5 cm away
from the side of the absorber.Besides the target and spark
,i
- 5 -
chambers,the incident protons had to traverse a total of
330 g/cm2 of iron , i.e.
	
three geometrical interaction
lengths or 24 radiation lengths. By means of the spark
chamber pictures one could distinguish between particles
which suffered their first interaction in the target and
those which did not. Further,by means of the above men-
tioned threshold discriminators,one could localize approxi-
mately the depth in which the first interaction of an
event took place in the iron absorber.
A Energ*l Calibration
Fig. 2 and 3 show some of our results. In Fig. 2 the meas-
ured average ionization energy is plotted versus the total
energy of the incident proton. ENi means N1 + N2 + N3,
where N1, N2, and N3 are the average particle numbers of
the two scintillators or each of the three measuring units
as recorded by MI, MII, and MIII, respectively. "One par-
ticle" is by our definition a pulse height at the output
of the PM tube corresponding to 1.15 times the most prob-
able pulse height generated by a single minimum ionizing
cosmic ray moon traversing the apparatus at sea level.
One can see from the graph that within the statistical
error the dependence of the average measured value of E Ni
on primary energy is linear if the primary particle did
not interact above the spectrometer, but is not linear if
only events are selected which show an interaction in one
of the targets. Moreover, there seems to be a much larger
fraction of energy unsampled in the latter case.
Fig. 3 shows for the same groups of selected events the
relative standard deviation of the distribution of the
measured parameter. One can see that target interactions
exhibit rather large fluctuations changing with primary
,,aergy, while the group of events with first interactions
- 6 -
between iron slabs a and e, inclusively, exhibits relative
standard deviations rather insensitive to any change in
primary energy. This can also be said of the shapes
of the corresponding distributions which are not shown
here.
If one plots one of the sets of experimental points of
Fig. 2 and takes one standard deviation of the corres-
ponding distribution up and down from that point as the
limit of error and connects the ends of the error bars,
then reading the graph with the ordinate as base line in-
stead of the abscissa yields an estimate of the error ex-
pected in measuring the unknown primary energy of an inci-
dent particle. This procedure yields approximate errors
for group (a)ofFig. 2 of +60% and -30% at about 10 GeV
incident energy, +50% and -30% at about 20 GeV , and
+50% and -25% at about 28 Gee'
	
The respective values
for group (b) of Fig. 2 are +80% and -35%, +65% and -30%,
and +60% and -30%.
B) Interaction Length of Protons in Iron
Although with some error, it was possible to locate the
first interaction of an incident particle in one of the
iron blocks of the spectrometer. This was done by means
of the "two particle threshold", the lower threshold of
T1 to T6 mentioned above. There are several possibilities
for mislocation of the first interaction of an event.
Apart from the overlap of the one-particle-distribution
of pulse heights over the threshold-,there can occur back-
scattering of particles from fast developing cascades and
also low multiplicity interactions which do not yield two
or more ionizing particles in the following scintillator.
While for the overlap of the distribution can be correc-
ted, the magnitude of the other two effects is rather
4
- 7 -
unknown. However, if in all cases of the latter two ef-
fects the first interaction is misplaced by a constant
number of iron blocks, it can easily be shown that the
slope of the distribution of beginning points of cas-
cades is not distorted if one retreats sufficiently far
from the upper and lower boundaries of the spectrometer.
In the present case it appeared that retreating one iron
slab from the upper and lower boundaries was sufficient
at 20.5 and 28 GeV/c. At 1 0 GeV/c the remaining points
of the distribution of *beginning points of cascades did
not appear to justify a straight line fit on a semi-log-
arithmic diagram, and therefore the interaction length
for protons in iron reported here is derived from the
data at 20.5 and 28 GeV/c only. An average of all values
of the slopes of the distribution of beginning points at
20.5 and 28 GeV/c incident protons yielded for the inter-
action length of protons in iron Lp
-Fe 
= 1.39+59
	 cor-
responding to an interaction cross section of
6-= 666+22 mb.p Fe
C) Inelasticity
Although the distributions of inelasticities at different
energies and for different materials could not be deter-
mined, we estimated mean values of the inelasticity for
the three materials carbon, iron, and lead).
Our results are shown in Fig. 4. The curve drawn is the
function
(1)	 R	 R(A) = 1 - (1 - n-n) 	 OC- Al /3
where "R is the mean inelasticity and A the Atomic Mass Number.
This equation was first put forward during the Jaipur con-
ference on cosmic rays5) . The curve was normalized to our
value for carbon. We could not find a significant energy
- 8 -
dependence in the range between 10 and 28 GeV primary
energy. So the reported values are averaged over the three
energies in the case of iron and carbon and over 20.5 and
28 GeV for lead. For lead at 10 GeV the inelasticity
could not be determined.
IV. Discussion
Our results of this experiment indicate that in principle
it is possible to measure the energy, although with a
rather large error, of a primary proton with an ionization
spectrometer only 3 geometrical interaction lengths in
depth. Also i* was possible to estimate the inelasticity
of nuclear interactions and the cross section for inter-
actions with subsequent cascade development. To get an
estimate of the latter two entities during balloon flights
will not be possible with such a shallow spectrometer
since these a- ,e
 
the entities which determine the energy
measurement of the event.
The most significant result is probably the knowledge of
the distributions of the parameter Z Ni and their insensi-
tivity against change in primary energy. Since the largest
contributions to the width of the distributions are ex-
pected to be the poisson distribution of the number of
interactions in the spectrometer and the inelasticity
distribution, which from present knowledge are not sup-
posed to change rapidly with energy, we expect that these
distributions do not change significantly in width and
shape for at least one more order of magnitude in primary
energy. Thus by finding a reasonable extrapolation for the
energy dependence of the mean values of Z: Ni,it is pos-
sible to measure the spectrum of cosmic rays even with a
shallow spectrometer like the one described here, although
the energy measurement of individual events cannot be done
very accurately.
- 9 -
Acknowledgement
We would like to thank the Brookhaven National Laboratory
for permission to do the calibration measurements at the
AGS. We are indepted to the staff of the 80" bubble cham-
ber and to the staff o:' beam No. 4 for their help and
support during the exposure.
t
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References
1) N.L. Grigorov, V.S. Murzin, and I . D. Rapoport;
Zh. Eksp. Teor. Fiz. 34, 506, 1958. Sov. Phys.
JEPT 7, 348, 1958 •
2) K. Pinkau and K.V. Thompson; Rev. Sci. Instr.
37, 302, 1966.
3) K. Pinkau, U. Pollvogt, W. Schmidt, and R.W.
Huggett; Proceedings of the Ninth International
Conference on Cosmic Rays, London, 1965, Vol. 2,
p. 821.
^+)	 W.V. Jones, K. Pinkau, U. Pollvogt, W.K.H. Schmidt,
and R.W. Huggett; Some Properties of Nuclear
Interactions Obtained with an ionization Spectro-
meter at Proton Energies from 10 to 28 GeV. To be
submitted to Nucl. Inser. Meth.
5) S.A. Azimov, A.M. Abdullaev, V.M. Myalovsky, and
T.S. Yuldashbaev; Proceedings of the 1963 Inter-
national Conference on Cosmic Rays, Jaipur, India,
Vol. 5, p . 69.
6) N.A. Dobrotin and S.A. Slavatinski, Proceedings of
the Tenth Annual Rochester Conference on High Energy
Nuclear Physics, 1960.
^^^	 7)	 I.N. Erofeeva, L.G. Mitschenko, V.S. Murzin, I.D.
Rapoport, and L.I. Saricheva; Proceedings of the
Ninth International Conference on Cosmic Rays,
London, 1965, Vol. 2, p. 833•
- 10
- 11 -
Figure Captions
Fig. 1
	 Schematic diagram of the apparatus describe?
here.
Fig. 2 : The mean energy loss measured in units of E Ni
as a function of the primary total energy for
various groups of events: (a) first interac-
tion in Fe(a); (b) first interaction in Fe(a)
to Fe(e); (c) all events without target inter-
actions; (d)first interaction in C-target;
(e) firs interaction in Pb-target. The size
of the symbols represents the errors of f Ni.
Fig. 3	 Relative standard deviations (7/ M as func-
tions of the primary total energy for the same
groups of events as in Fig. 5.
Fig. 4	 Comparison of experimental values for the
average total inelasticity K with the rela-
tion (1) of the text. The dashed error bars
represent 15% errors arbitrarily attached to
the values for which no errors were quoted
by the original authors. The value of R de-
termined by this experiment, as well as for
some of the other quoted results, is taken
to be 3 R7ro. The curve is normalized to the
value 'R = 0.45 obtained in this experiment
for the carbon target.
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Properties of an ionization spectrometer exposed to 10, 20.5, and 28 GeV/c machine accelerated protons

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