The techniques to be used in a balloon borne experiment APEX to detect  220 MeV antiprotons are described, paying particular attention to potential sources of background. Event time history is shown to be very effective in eliminating this background. Results of laboratory tests on the timing resolution which may be achieved are presented

Acharya, B. S.

Balasubrahmanyan, V. K.

Lloyd-Evans, J.

Ormes, J. F.

Stephens, S. A.

Streitmatter, R. E.

English

NASA Technical Reports Server

254 OG 9.1-1
THE DESIGN OF AN EXPERIMENT TO DETECT LOW ENERGY ANTIPROTONS
J. Lloyd-Evans*, B. S. Acharya+, V. K. Balasubrahmanyan
J. F. Ormes, R. E. Streitmatter, and S. A. Stephens_
NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A.
ABSTRACT
The techniques to be used in a balloon-borne experiment
("APEX") to detect < 220 MeV antiprotons are described,
paying particular attention to potential sources of
background. Event time history is shown to be very
effective in eliminating this background. Results of
laboratory tests on the timing resolution which may be
achieved are presented. The limiting experimental
sensitivity of this experiment is -#/p< I0-v.
1. Introduction. The conceptual design of a balloon-borne experiment
to detect low energy antiprotons in the cosmic ray flux has been
described by Balasubrahmanyanet al [I]. The serious revision of ideas
concerning the origin and transport of galactic cosmic ray protons
seemingly required by the high p-flux reported by Buffington et al [2]
makes such an independent measurement at solar minimum an urgent
requirement.
Referring to Figure 1, the trigger criterion of:
SI(> 1.6 min.)* $2(> 1.6 min.)* C-I (prompt)* C2 p_rmits the
unambiguous identification of anti protons if (a) p/p > i0 -_ and (b)
event time history in OS ('outershield') can be measured to O.Sns. We
have previously considered [I] the importance of vetoing protons > 220
MeV because of the rapid rise in the_° production cross section above
this energy [3]
Here we co_sider other backgrounds in detail (Section 2), and show
how the timing history is effective in eliminating all but the neutral
high energy background. Schemes to achieve the timing resolution are
discussed in Section 3, and the results of laboratory tests reported in
Secti on 4.
2. Background Elimination. The flux, coincidence rate, and method of
elimination for each type of background are summarized in Table I.
These are:
(a) _° production by protons > 220 MeV--the elimination of this
background has been discussed in reference [I].
(b) An out-of-geometry relativistic charged particle
interacting in the Pb glass calorimeter (C2) and depositing an energy :
annihilation energy in time coincidence with a slow (< 220 MeV) proton
within the geometry. This is the.most serious background, amounting to
75% of the _/p ratio at 2 x 10-4 . The time history (OS precedes C2 by
3ns) is to be compared to that for a genuine _ event (C2 precedes OS by
3ns) so that this background can be eliminated (to-p/p < I0 -_) if (I)
* Also University of Maryland, Department of Physics and Astronomy
College Park, MD 20742, U.S.A.
+ NAS/NRC Research Associate
Tata Institute of Fundamental Research, Bombay, INDIA
https://ntrs.nasa.gov/search.jsp?R=19850026615 2020-03-20T16:48:30+00:00Z
255 OG 9.1-1
minimum ionlzing particle detection efficiency is > 99% and (2) the
relative times in $1, C2, and OS can be measured to o(At) : 0.5 ns.
Such timing resolution (nominally a '12o' rejection factor) is
deliberately conservative in recognition of the existence of non-
Gaussian tails in scintillator timing distributions.
(c) OS is ineffective if the out-of-geometry particle in (b) is
an atmospheric neutron. Calculations have been performed including the
measured n_utron flux [4], the expected directional distribution at
2-5 gm cm"_ of residual atmosphere, the interaction cross section, and
experimental geometry factor. We determine that the spurious p-events
induced by atmospheric neutrons correspond to a-#/p ratio < 2 x 10-°.
Since this background cannot readily be eliminated, it determines
the _/p sensitivity of a calorimetric balloon-borne detector in this
energy range.
Table 1. Summary/of APEX Backgrounds
Text Particle Flux/cm2sr.s. Ref. Backgr_)und Elimination
Ref. s--
H.E. p > 6 GeV: < 6xlO -Z [ 9] 2xlO -3 Timing
(c) n > 2GeV: < 3xlO -3 [ 4] 8xlO -6 --
> 1GeV: _ 5xlO -J [ 4] 2xlO -b --
Idel e- > 0.I GeV: ixlO-_ 2 [i0] 3xlO -4 Timingy . I : < 6xl -b [i ] 2 l 0 --
(f) p+e < 50MeV: < 104 [Ii] < 4xlO -4 C2 Threshold
I
Figure I. (left) APEX
Design. CI, C2: Crown
Glass, Lead Glass Cherenkov
Detectors. HI, H2:
Hodoscopes. Sl, $2, IS,
OS: Scintillator
(d) Out-of-geometry
electrons > 100 MeV, again
in time coincidence, result
in a cascade in C2 which
could simulate an anni-
hilation. This background
is eliminated by timing as
efficiently as (b)
(e) Atmospheric
y-rays (> 100 MeV) cannot
be rejected efficiently
I_M_T_,_ I by OS, and can simulate an
annihilation as in (d). Fortunately, the flux of a_:mospheric y-rays is
low, resulting in a < 2% contribution to _/p at I0 -_.
(f) A low energy (< 50 MeV) atmospheric muon (satisfying the
within-geometry criteria) can decay in C2. although the rate is high,
the resulting cascade is low energy. Simulations have shown that ~ 95%
256 OG 9.1-1
of the resulting C2 signals are below 98% of the _ signals, so a
suitable C2 threshold can effectively eliminate this background.
Summarizing,the time history eliminates all but the neutral
backgrounds, resulting in a sensitivity of-p/p --10 _. An experiment
above the atmosphere would achieve much better sensitivity because of
the virtual absence of the atmospheric n, y background.
3. Timing Resolution. To achieve the high detection efficiency and
good timing resolution,we consider using a 1 cm thick segmented
scintillator sphere of 1.2m radius completely surrounding C2. Two
possibilities are considered.
(1) Approximately 250 x 2" PMT's in optical contact with the
scintillator. A large number is required to satisfy the conflicting
requirements of redundancy (each signal seen by > 2 PMT's) and a high
order of scintillatorsegmentation. For a background event, the signal
in some of the 0S PMT's consists of that of the interaction progenitor
followed (in a barely resolvable time) by that of the interaction
products. Positional information is therefore _ifficult to extract
The segmentation area (each PMT viewing ~ 0.1 mL) is thus dictated by
the acceptable time dispersion due to light propagation (at C/2) across
the whole scintillatorsegment.
(2) Segmentation can be relaxed by viewing the scintillator face-
on. Simulationshave shown that a sphere of~ 50 x 5" PMT's, looking in
towards 0S provide sufficient redundancy, photoelectronefficiency (> 10
p.e.) and minimum time dispersion. This sphere is located~ 40 cm
outside OS so that direct Cherenkov emission in any one PMT does not
provide a false veto, which would reduce the genuine pdetection
efficiency.
Design (2) is to be preferred on the basis of cost-effectiveness,
if 5" PMT's can be demonstratedto have the required timing stability.
4. Timing Results. Preliminary tests on RCA 8575 (2") and 4522 (5")
ha_ been performed with a small (5cm x 5cm x O.5cm) scintillator and
Sr source. The resulting signal distribution is not dissimilar in
amplitude (thresholdof~ 10 p.e.) or, more importantly, dynamic range
(~ 100:1) to that expected in practice. The standard start-stop, low-
high technique has been employed with two PMT's viewing the scintillator.
Application of an unconventionaltiming technique [5] allows the
measurement of both time and amplitude in a Single commercial ADC module
(LeCroy 2249A). Excellent resolution (o(At) < 30 ps) and linearity
(integral non-linearity over a dynamic range_f 50:1 of < 0.5% full
scale) for the recording electronics is in agreement with the findings
of Venema [5].
Figure 2 shows a comparison of the timing resolution achieved
(o(At) : V-_X o(ti)) for the 2" and 5" PMT's, for leading edge and
constant fraction discriminators (Ortec 453) as a function of the
fraction of the low to high discrimination levels. The leading edge
resolution is inadequate without time-walk corrections. In agreement
with D'Agostini et al. [6], we find that an _ividual time correction
z/c
to each PMT of t_-eArm At. = K./(amplitude) minimizes the time
dispersion, and that the c_rrec_ed resolution is comparable to that of
the constant fraction discriminator.
257 OG 9.1-1
' ' ' ' ' Ftgure 2. (left) Time
Resolution for leadtng edge
700 _J discriminationwithout walk
j_J correction (e 2" PMT); withwalk orrection (x 2" PMT,O600
j_y_ 5" PMT); and constant
fraction (o 2" PMT).5(X)
a(AT)
PS 4O0 ,f_Z a _'P The resolution
.......__,____ achieved for the 2" PMT
300 _ _ _ , does not rival other
_ reports (in particular,
200 reference [6]) but, for
both 2" and 5" PMT's, it is
1oo sufficient for the purposes
outlined in Section 3.
0 A I I A A
o _o 20 30 40 5o 60 5. Conclusions. The
background limitations for
P.H.FRACTION %
APEX have been described.
Severe demands are made on the time resolution of a large area
scintillator shield in order to eliminate this background. Preliminary
tests on small scintillators show that this resolution can be achieved
with 2" or 5" PMT's. We are currently investigating much larger area
scintillators. As an alternative, we are also investigatingthe
possibility of using large area PSC's [7] as shields. These detectors
have unprecedented time resolution (< 50 ps), offer the possibility of
particle track identification,and (because of the localization of
discharge) are sensitive multihit devices.
6. Acknowledgements. We thank J. Linsley for pointing out the timing
characteristics of PSC's.
References
I. Balasubrahmanyan, V. K., et al. (1983), 18th ICRC, Bangalore,
Special Session on "p and other antiparticles".
2. Burlington, A., et al. (1981), Ap. J. 248, 1179.
3. Stephens, S. A., (1981), 17th ICRC, Pa_, 9, 60.
4. Hess, W. N. et al. (1961), J. Geo. Research, 6__66,65.
5. Venema, W. Z., (1984), NIM, 219, 257.
6. D'Agostini, G., et al. (1981,]-_NIM, 185, 49.
7. Atwood, W. B., et al. (1983), NIM, 2-06_,99.
8. Balasubrahmanyan,V. K., et al. (19-6_T,J. Geophys. Res, 7_1_1,771.
9. Ginzburg, V. L. and Syrovatskii, S. I. (1964), "The Origin of
Cosmic Rays" (McMillan).
10. Daniel R. R., Stephens, S. A.(1974), Revs. Geophys. Space Phys.,
12, 233.
11. Stephens, S. A. (1981), 17th ICRC, Paris, 4__,282.


The design of an experiment to detect low energy antiprotons

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