The rate of neutron interactions was measured for the energy range 7.5 to 60 MeV, using a 3.85 kg cell of liquid scintillator. The neutrons are selected by pulse shape discrimination, with anticoincidence counters used to reduce interference from muons transversing the scintillator. The observed flux is interpreted in terms of neutrons produced from environmental uranium and thorium, those resulting from the capture of negative muons in nuclei and those from fast muon interactions

Barton, J. C.

English

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•g8' HE 5.2-20
THE SPECTRUM OF NEUTRONS AT 60 hg m-2
J.C. Barton,
The Polytechnic of North London, London, N7 8DB, U.K.
ABSTRACT
The rate of neutron interactions has been measured in the Holborn
underground laboratory for the energy range 7.5-60 MeV, using a
3.85 kg cell of liquid scintillator. The neutrons are selected
by pulse shape discrimination, with anticoincidence counters used
to reduce interference from muons transversing the scintillator.
The observed flux is interpreted in terms of neutrons produced
from environmental uranium and thorium, those resulting from the
capture of negative muons in nuclei and those from fast muon
interactions.
i. Introduction. This paper describes the results obtained with an im-
proved version of the experiment reportedat the last conference (Barton,
1983). A preliminary attempt is made to estimate both the source
spectrum of neutrons at this depth and the proportion of these neutrons
which would be detected by the present system.
2. Apparatus. The counters are arranged as shown in Figure i. The
neutron counter is a cylinder of 175 mm diameter and length, containing
3.85 kg of NE213. The two anticoincidence counters are always operated
in parallel; some results have been taken with them used in coincidence
with the neutron counter. The 5 cm lead screen was removed for part of
the observations.
The data from the pulse shape discrimination circuits were recorded
as 2-dimensional histograms. Using an Am/Be neutron source, the dis-
crimination was found to be very satisfactory up to its maximum energy
(_ 12 MeV). Of course the discrimination becomes less .at higher ener-
gies but is adequate to beyond 50 MeV. The present arrangement was
less satisfactory at energies below 8 MeV because the photo-electron
statistics were poor; a larger diameter photomultiplier might, in
principle, improve the performance at low energies, but a tube with
sufficiently uniform time response was not available.
[ I I ^,
/, / / / / / / X] 1////,4,,,////,I •
I,.°.-1-',,-.
End view Side view
I I I I i I
0 O.5 m
Fig. i: Arrangement of apDaratus
https://ntrs.nasa.gov/search.jsp?R=19850027783 2020-03-20T17:03:14+00:00Z
99 HE 5.2-20
The discriminating properties of organic scintillators are asso-
ciated with their non-linear response to more heavily ionizing particles.
The position of the cosmic ray peak provides a direct calibration for
minimum ionizing particles, but the energy scale for protons is not so
easily established. There are no convenient calibration sources so the
energy scale had to be taken from reports of calibrations made on other
large NE213 counters used for accelerator experiments. The published
data are not in good agreement so the neutron energy scale adopted here
has an uncertainty of ± 15%.
The results have to be corrected for two types of spurious event.
The trigger levei for the leading-edge timing must be set low to avoid
time-slewing which means that accidentals cause spurious zero-crossing
times. Secondly, muons which stop in the counter may decay before the
zero-crossing time (1.5 _s was used) and give distorted events. Both
effects have been analysed theoretically and compared with the background
events in the pulse-height/pulse-shape histograms in regions away from
the muon or proton ridges. At worst the correction for the proton
region was 15% and the additional uncertainty is generally less than the
statistical one.
When operating the main counter in coincidence with the others, a
well-resolved proton ridge was not observed. This is understandable, as
the signal would be due to both muon and proton in proportions varying
from event to event. These events would fall in the valley region
between the two main ridges. Similarly, events with indications of
particles ionizing more strongly than protons are ascribed to nuclear
disintegrations in the scintillator. For the anticoincidence events
both effects were sufficiently small that the true proton events could be
separated with less difficulty.
3. Results. There was no clear difference between the anticoincidence
results with or without the lead in position. The combined results are
therefore used for Figure 2. The coincidence rates are also shown but,
as explained above, there is much greater uncertainty about these.
4. Predicted source spectrum of neutrons. All rocks contain a small pro-
portion of uranium, typically from 1-5 _g g-l, about three times as much
thorium and a few per cent of potassium. Neutrons can therefore result
from spontaneous fission of 238U, from (_,n) reactions and (y,n) reac-
tions. For a rock in the middle of the range indicated above, these
processes give, respectively, about 5 x 10-8 g-I s-l, 1.2 x i0-7 g-I s-I
and < 10-9 g-i s-l. These values depend on the other constituents of the
rock but not very strongly, except in particular ores. The spectra of
. neutrons from fission and (_,n) processes in thick targets are known to
fall off rapidly above 2 MeV.
The stopping rate of all muons at this depth is (21±2) x 10-3 g-1 d_
and for "standard" rock with Z = Ii about 0.5 x 0.5 will be captured
and produce, on average, 1.2 neutrons each (Mukhopadhyay, 1977) giving a
total yield 6 x 10-3 g-I d-1 The spectra given by Sundelin (1973)
show that about a figth of the neutrons have energy greater than i0 MeV
and decreasing % e-E/7.
IO0 HE 5.2-20
! ! ! ! | !
-.o
" process _ event rate in anticoincidence
I: , event rate in coincidence
Spontaneous
fission n'si
(Note that the effects of reaZ photons
are not included. They will contribute
# ' at least as strongly as the virtual Dho-
h0
N tons in the G.D.R. re_ion and, _ossibl¥,
• + also in the pseudo-deuteron re_ion.)T
. . . \
0.2 \ .Total neutron source function
4o
\ -I /'-
. Hadron cascade
%% k< " mrs", \
0.01- _ ",... Muon cadre n's -
---\--- ---....\  ou o- outo o\ -.
I \ I I I I I
0 i0 20 30 40 50 60 70
Neutron or event Energy MeV
Fig. 2: Observed event rate compared to neutron source soectra
The fast muons traversing the rock produce neutrons through both
real and virtual photo-nuclear interactions. For energies below i00 MeV
most of the real photons result from bremmstrahlung of knock-on electrons
and have a spectrum of the form % E-2. In the same energy region the
virtual photons have a spectrum _ E-I so, at sufficiently high energies,
will always be more important than the real photons. Although the
various electromagnetic interaction processes producing real photons are
well understood, the detailed Monte Carlo calculations required to
estimate the absolute intensity of photons with energy comparable to the
critical energy do not seem to have been carried out. For the spectrum
of virtual photons, the plane wave Born approximation has been shown to
be valid at low energies (Orth et a____l,1981), whilst at energies above the
pion production threshold the refined calculations of Bezrukov and Bugaev
(1981) are available.
HE 5.2-20
I01
Photo-nuclear reactions are usually considered in three energy
regions. Below 30 MeV the giant dipole resonance provides the mechanism
and the resulting neutrons have a spectrum similar to those from the
evaporation process. Between 30 and 150 MeV the pseudo-deuteron model of
Levinger has recently received more precise experimental support for
light and medium nuclei (Homma et al, 1983). The neutron usually re-
ceives half the energy of the photon, less the binding energy. Above the
pion production threshold neutrons are produced both directly in the
hadron cascade and from the capture of negative pions by _uclei. The
spectrum of the neutrons in the cascade falls off as _ E-_ between i0 and
50 MeV but more rapidly at higher energies (Metropolis eta l, 1958), with
the absolute values estimated from the total amount of energy transferred
via muon nuclear processes. Negative pions produced by the same mech-
anism are captured by pseudo-deuterons in nuclei and produce one or two
neutrons with a rather flat spectrum up to _ i00 MeV (Madey etal, 1982);
the number of pions stopping at 60 m.w.e, has been determined experimen-
tally (Slade, 1966). Figure 3 includes estimates of the main contribu-
tions to the neutron source spectrum.
In the present experiment, as was pointed out earlier, only the
anticoincident results enable the neutron events to be separated un-
ambiguously. Those neutron sources in which the originating muon is
closely collimated with the neutron will therefore be excluded. A fur-
ther complication is that the neutrons may be scattered and lose energy
before reaching the detector. Overall, it must therefore be expected
that the observed anticoincidence spectrum will be softer than the
source spectrum and substantially lower in magnitude. Examination of
Figure 2 shows this to be true but it is not yet possible to say whether
the difference can be accounted for quantitatively.
5. Conclusions.
i. The observed spectrum of neutron events at 60 hg cm-2 falls sharply
up to 15 MeV and then decreases rather slowly.
2. The observed intensity of neutron interactions, not closely accom-
panied by muons, above 15 MeV at this depth is 1.8 ± 0.3 kg-I d-I •
3. The total intensity is at least double this value and is not in dis-
agreement with what can be predicted from known processes.
References
Barton, J.C., (1983), Proc. of 18th Int. Cosmic Ray Conf., Bangalore,
11,462-5
" Bezrukov, L.B. and Bugaev, E.V., (1981), Proc. 17th Int. Cosmic Ray
Conf., Paris, 8, 90-3
Homma, S., Kanazawa, M., Maruyama, K., Murata, Y., 0kuno, H.,
- Sasaki, A. and Taniguchi, T., (1982), Phys. Rev. C., 27, 31-45
Madey, R., Vilaithong, T., Anderson, B.D., Knudson, J.N., Witten, T.R.,
Baldwin, A.R. and Waterman, F.M. (1982), Phys.Rev.C., 25, 3050-67
Metropolis, N., Bivins, R., Storm, M., Miller, J.M., Friedlander, G.
and Turkevich, A., (1958), Phys. Rev., ii0, 204-219
Mukhopadhyay, N.C., (1977), Phys. Rep., Phys. Lett. 30C, 1-144
0rth, C.J., Knight, J.D., Wolfsberg, K. and Johnson, M.W., (1980),
Phys. Rev. C., 21, 1967-73
Sundelin, R.M. and Edelstein, R.M., (1973), Phys. Rev. C., 7, 1037-60
Slade, M., (1966), Ph.D. thesis, University of London


The spectrum of neutrons at 60 hg m(-2)

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