The positron data gathered in conjunction with electron data published elsewhere is reported. The basic recognition scheme was to look for low mass positive particles that cause a cascade in a 7 radiation length shower counter. The mass criteria is imposed by selecting particles that were accompanied by Cherenkov light but whose rigidity was below the proton Cherenkov threshold. Thus the proton Cherenkov threshold represents an upper limit to the range of the experiment

Badhwar, G. D.

Daniel, R. R.

Golden, R. L.

Horan, S.

Lacy, J. L.

Mauger, B. G.

Stephens, S. A.

Zipse, J. E.

English

NASA Technical Reports Server

Observation o f  Cosmic Ray Posi t rons 
From 5 t o  25 GeV 
R. L. Golden, B.G.Mauger, and S. Horan 
Department o f  E l e c t r i c a l  and Computer Engineering 
New Mexico State  Un ivers i t y  
Las Cruces, NM 88003 
USA 
G.. D. Badhwar, J. L. Lacy, J. E. Zipse 
NASA, Johnson Space Center 
Houston, T X  77058 
USA 
R. R. Daniel, S. A. Stephens 
Tata I n s t i t u t e  For Fundamental Research 
Homi Ehabha Road 
Bombay, Ind ia  
1 Introduct ion.  
-- ------------- We repor t  here the pos i t ron data gathered 
i n  con junct ion w i t h  e lec t ron data pub1 ished elsewhere (1  1 .  
The basic recogni t ion scheme was t o  look f o r  low-mass 
p o s i t i v e  p a r t i c l e s  t h a t  cause a cascade i n  a 7 rad ia t i on  
1 ength shower counter. The m a s s c r i t e r i a  i s  imposed by 
se lect ing p a r t i c l e s  t h a t  were accompanied by Cherenkov , 1 i g h t  
but  whose r i g i d i t y  was below the  proton Cherenkov threshold. 
Thus the proton Cherenkov threshold represents an upper l i m i t  
t o  the range of  the  experiment. 
The Apparatus. The p r i nc ipa l  detector elements are (from 
--- 
top t o  bottom) : a gas Cherenkov detector ( G ) ;  p l a s t i c  
s c i n t i l l a t o r s  S1 and S2; 8 mul t iw i re  proport ional  counters ( M I -  
M8); and a l e a d - s c i n t i l l a t o r  shower counter comprised of 7 
layers. Each layer  o f  the  shower counter consisted of 1 
s c i n t i l l a t o r  and 1 r a d i a t i o n  length of lead (PI-P7). The 
mu1 t i  wire propor t iona l  counters (MWPC) were arranged i n  three 
p a i r s  located a t  the top, middle and bottom of the spectrometer 
w i th  the remaining two chambers located a t  the 1/4 and 3 /4  po in ts  
i n  the spectrometer stack. A11 phototubes were pulse-height 
analyzed, 8 measurements were made of  the p a r t i c l e  pos i t i on  on 
the x ax i s ( ax i s  o f  de f lec t ion )  , and 5 measurements were made 
on the y axis. Data readout was i n i t i a t e d  f o r  every occurence 
of  an Sl*Pl*P7 coincidence. The geometric fac to r  of the 
instrument was 324t5 cmA2-str and the  l i v e  t ime f r a c t i o n  was 
0.80. The data were gathered on a bal loon f l i g h t  from 
Palest ine Texas on May 20,1976, The data gathering per iod was 
6.4 x lOA4 seconds a t  an average a l t i t u d e  corresponding t o  5.8 
gm+cmA-2. 
3, Data Analysis. Select ion o f  the  pos i t rons s tar ted wi th  
using the same c r i t e r i a  used f o r  the  e-: 
1) MWPC data. There must be a t  l eas t  5 v a l i d  MWPC readouts 
f o r  the X coordinate, and 3 v a l i d  readouts f o r  the Y axis. The 
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reconstructed t r a j e c t o r y  m u s t  f i t  t o  a va l id ,  continuous track 
w i th  a chi-squared of l ess  than 50 i n  the x ax is  and 30 i n  the 
Y axis. And f i n a l l y ,  one MWPC from each o f  the p a i r s  have 
va l  i d  readouts. 
2) Charge. The average of  the pulse-heights for  
s c i n t i l l a t o r s  S1 and S2 correspond t o  less  than 1.8 I, where 1,is 
the pulse-height f o r  a Z= l ,  9=1 ve r t i ca l1  y inc ident  par t i c le .  
3) Cherenkov Detector. The Cherenkov detector be above a 
discr iminator l eve l  corresponding t o  0.25 photo-el ectrons. 
4) Shower Counter. The sum of the shower-counter pulse- 
heights correspond t o  a t  leas t  50 I, 
C r i t e r i a  1 d i f f e r s s l i g h t l y  f r o m t h a t  used i n  (1). Theo ld  
MWPC c r i t e r i a  required tha t  the bottom MWPC p a i r  both have v a l i d  
readouts. This was replaced wi th  the c r i t e r i o n  that  each pa i r  
must have a v a l i d  readout. I t  was found that  the revised 
c r i t e r i o n  has a 13% higher e f f i c i ency  without any degredation of 
resolut ion. 
The c r i t e r i a . 1 - 4  y i e l d  a sample which i s  roughly 80% protons 
and 20% positrons. The main source of  background i s  the high 
noise leve l  i n  the Cherenkov detector- In  order t o  reduce the 
background we requ i re  a minimum Cherenkov pulse-height 
corresponding t o  about 1 photoelectron. The Cherenkov mirror 
was divided i n t o  4-quadrants, each viewed by a separate 
phototube. Phototubes from opposite quadrants were summed 
before d ig i t i za t i on .  Trajectory analysis was used t o  determine 
which quadrant p a i r  should have registered the l i g h t .  I t  was 
demanded that  the correct  p a i r  have at  leas t  the minimum pulse- 
height, and the other p a i r  have less than the minimum pulse 
height. This se lect ion i s  ca l led  c r i t e r i o n  5- Application of 
these c r i t e r i a  t o  the e- sample revealed tha t  93% passed the 
test ,  and an estimated 75% of the remaining protons were 
rejected. 
The next c r i t e r i o n  was t o  examine the shower counter output. 
Each set of  Pl-P7 outputs was f i t t e d  t o  the hypothesis that 
there was an electromagnetic cascade- The s ta r t i ng  point 
and energy of the shower were used as unconstrained 
var i abl es i n  the f i t .  C r i t e r i on  6 was tha t  the shower f i t  
have a chi-square of  less than 10. Once again i t  was observed 
that  93% of the e- pass t h i s  test ,  and i n  t h i s  case about 36% of 
the remaining protons were rejected. 
Figure l a  shows the de f lec t ion  ( l / r i q i d i t y )  spectrum of the 
events selected by c r i t e r i a  1-4. We have include both pos i t i ve  
and negative def lect ions (corresponding t o  pos i t i ve  and negative 
charged par t i c les ) .  The peak near zero de f lec t ion  i s  due 
t o  protons above Cherenkov threshold. The gradual r i s e  
wi th  increasing posi t i  ve de f lec t ion  i s  due t o  the 
combination o f  posi t rons and protons accompanied by an 
accidental G pulse. The dec l ine above 0.2 c/GV i s  due t o  the 
geomagnetic cu to f f  . The events t o  the  l e f t  of  zzro def lect ion 
are the e-. Note tha t  the corresponding geomagnetic cutof f  
i s  smeared due t o  bremsstrahlung losses. Figures l b  and 
l c  show the progressive e f f e c t s  of  c r i t e r i a  5 and 6. The peak 
due t o  protons above Cherenkov threshold i s  broadened by 
the f i n i t e  reso lu t ion  of the  r i g i d i t y  resolut ion,  We have 
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chosen the upper l i m i t  f o r  t he  experiment t o  be 0.04 c/GV which 
i s  1.5 standard dev ia t ions below the proton Cherenkov 
threshold. With t h i s  r e s t r i c t i o n  i t  i s  possib le t o  generate a 
sample o f  protons by applying the  e+ se lect ion c r i t e r i a  except 
demanding t h a t  no G pulse occur. Given t h i s  sample o f  protons, 
and the e- i t  i s  possib le t o  perform quan t i t a t i ve  evaluat ions o f  
the remaining background i n  the  e+ sample. 
!%sure 1 
Ef fec t s  of 
F'osi t r on  
Sel ect i on 
C r i t e r i a  
The s t a r t i n g  po in t  d i s t r i b u t i o n s  f o r  protons, e- and our 
candidate sample o f  e+ w i l l  be shown a t  the conference. The e- 
d i s t r i b u t i o n s  are peaked sharp1 y a t  about +1 rad ia t i on  length. 
This i s  as expected f o r  e lectrons (and posi t rons) except 
t ha t  the peak i s  s l i g h t l y  o f f s e t  due t o  an a r b i t r a r y  fac to r  
i n  the shower f i t t i n g  program. The proton d i s t r i b u t i o n s  
r e f l e c t  a bas i ca l l y  f l a t  i n te rac t i on  p r o b a b i l i t y  biased by the 
c r i t e r i a  t ha t  there b e a t  leas t  a 5 0  I, shower sum. The e+ 
candidates show tha t  the sample i s  comprised of mostly electron- 
l i k e  events w i th  a small admixture of protons except a t  h igh 
energies, where the proton component dominates. 
A background subtract ion was made on the assumption tha t  a l l  
e+ would have s t a r t i n g  po in t s  between +3.5X0 and -2.5X0 . I f  
one counts events outs ide t h i s  region i n  the proton 
d i s t r i bu t i ons ,  one can form a background m u l t i p l i c a t i o n  
fac to r  M = ( t o t a l  protons) / (protons outside the e lec t ron - 
s t a r t i n g  po in t  l i m i t s ) .  Each r i g i d i t y  i n te r va l  i s  then 
analyzed and the number o f  e+ i s  determined by the formula 
#e+ = (#candidates ins ide  1 i m i  t s )  -M*(#candidates outside 1 i m i  t s )  . 
Table 1 gives some of the deta i  1s of the background subtract ion 
.- 
process f o r  e+. Table 1 a lso gives the number of e- determined 
by applying c r i t e r i a  1-6 and performing a s im i l a r  background 
subtract ion. 
Tdef lec t io r  
Table 1. Background subtract ions 
e+ I Proton I Atmosl e+ w/o I e- w/o i 
- 
4: R~~u122: Reference 1 contains the d e t a i l s  o f  the 
exposure f ac to rs  ( s o l i d  angle x area x t ime x ef f ic ie, rcy)  f o r  
e- during the f l i g h t .  The exposure fac to rs  f o r  the  e+ 
c r i  t 1-6 
742 15 
51 10 
60 6 
55 5 
54 1 
28 0 
bkgrnd 
69.6 f: 19.5 
41.5 13.8 
22.3 9.4 
19.2 8.8 
3.8 3.8 
0.0 4.3 
bkgrnd 
2.8 
4.7 
7.1 
8.1 
10.8 
13.3 
bkgrnd 
1.6% 16.5 
4.8 12.0 
30.6 10.2 
27.7 9.9 
39.4 8.5 
14.7 7.3 
bkgrnd 
1325 15 
304 18 
377 18 
412 21 
268 17 
137 13 
OG 6.2-5 
376 
chosen the upper limit for the experi ent to be 0 .  c/ which 
is 1.  standard deviations below the proton Cherenkov 
thr shol . ith this restriction it is possible to generate a 
sample of protons by applying the e+ selection criteria except 
demanding that no G pulse occur. Given this sample of prot , 
and the e- it is possible to perform quantitative evaluations of 
the remaining background in the e+ sampl . 
(. E!.g!:!!:g 1 
Effects of 
Positron 
Selection 
riteria 
400 400 
8 
CCUT ... SNOWU CUT 
·.2 -. 1 0 .1 . 2 
""CMETIC aleolOITY <C'CU) 
- .1 -.1 a . 1 . 1 
""CHElIe IrCIOITY (C'C-" 
The starting point distributions for prot s, e- and our 
candidate sample of e+ il  be shown at the conf . The e-
distributions are peaked sharply at about +1 radiation length. 
This is as expected for electrons (and positrons) except 
that the peak is slightly offset due to an arbitrary factor 
in the shower fitting progra . The proton distributions 
reflect a basically flat interaction probability bi sed by the 
criteria that there be at least a 0 10 shower su . The e+ 
candidates show that the sample is comprised of ostly electr
like events with a small admi x ture of protons except at high 
energi s, where the proton co ponent do i t . 
A background subtraction was made on the assumption that al  
e+ would have starting points bet een +3.5X o and - . o If 
one counts events outside this region in the proton 
distributi s, one can form a background ultiplication 
factor M = (total protons) I (protons outside the electron 
starting point li its). Each rigidity interval is then 
analyzed and the number of e+ is determined by the for ula 
#e+ = (#canoidates inside limits)-M*(# i outside limi ). 
Table 1 gives so e of the details of the background subtraction 
process for e+. Table 1 also gives the number of e- determined 
by applying criteria 1- and performing a similar background 
subtraction. 
Tabl e 1. Background subtractions 
deflection e+ Proton At os e+ wl  e- wl  
c/GV crit 1- bkgrnd bkgrnd bkgrnd bkgrnd 
0.04-0.08 74 ! 15 6 = 19. 2.  1. ~ 16. 132!.. 15 
0.08-0.12 51 10 4  13. 4  4.  12. 304 18 
0.12-0.16 60. 6 2 . 9. 7.  3 . 10. 377 18 
0.16-0.20 55 5 19. 8. 8.  2  9. 412 21 
0.20-0.24 54 1 3. 3. 10.  3  8.  268 17 
0.24-0.28 28 0 0. 4.  13. 14. 7. 137 13 
1~ Bg§!:!!t§~ Reference 1 contains the details of 
area x ti e x ef iciedcy> 
exposure factors for the 
exposure factors (solid angle x 
e- during the fli t. The 
the 
for 
e+ 
observation are obtained by using the e- data i n  the rightmost 
column i n  Table 1. The r a t i o  of  e- i n  Table 1 t o  the number 
i n  (1) gives a correct ion t o  the exposure factor  reported i n  
(1). f able 2 gives the computed exposure factors  and fluxes. 
Reference 1 gives a detai led discussion of the corrections f o r  
geomagnetic cutof f ,  solar modulation and bremsstrahlung energy 
losses. I n  (1) a mean I S M  energy ( E ), an equivalent b i n  width 
( A E ) and a propagation e f f i c iency  (p) are computed fo r  each 
def lect ion in te rva l  i n  Table 1. These factors  are also given i n  
tab le 2. 
Table 2 Positron Flux Calculat ions 
-----
v 
References 
(1) Golden, R. L. e t  a l .  (1984). Ap. J. 282, 622. 
(2) Ste~hens. S. A. (1981), Proc 17th ICRC (Paris) 4, 282. 
Figure 2 shows the f luxes as determined i n  Table 2, along with 
the resu l t s  of other observers, and the f luxes predicted fo r  a 
simple leaky box (wi th energy dependendt leakage) and 7 gm/cmA2 
of matter traversed. 0 least  squares f i t  t o  the f luxes i n  tab le 
2 gives a spectral index of -4.2'0.6 and an in tegra l  f lux  (above 
5.2 Gev) of (0.51t 0.09) e+/mA2-str-sec-G~V. The chi -square fo r  the 
f i t  was 2.1. 
(3) ~ u f f  ingt&n, A. e t  al.  (1975), Ap. J. LYP, 669. 
(4) Faneslow, J. L. (1969). Ap. J. 13s. 771. 
d 
F 
( GeV ) 
21.7 
1 . 1  
9.39 
7.37 
6.08 
5.19 
l o2 [  
ti3 Eigure_ 2- r~ 
Positron Fluxes 
(U 
< 
W 
?+ 
101 
Thls Work S 6 2 
+ Buff~ngton e t  a[. (31 L, 
fB Faneslow et al. (4) 
2 leaky box 7 gm/cmA2 
1 oO 
- 
- 
- 
- 
- 
- 
L-- 
- 
- 
!! 
- 
I 
e+ 
counts 
1.6t 16.5 
4.8) 12.0 
30.6% 10.2 
27.7: 9.9 
39.4 z 8.5 
14.7: 7.3 
1 o0 lo1  1 02 
ENERGY (GEU) 
p 
1.05 
1.02 
0.99 
0.71 
0.43 
0.18 
LlE 
( GeV ) 
14.5 
5.13 
2.61 
1.57 
1-02 
0.75 
e+/ (e++ e-) 
0.01 t 0.12 
0.02: 0.04 
0 . 0 8 ~  0.02 
0 . 0 6 ~  0.02 
0.13 r 0.03 
0.10t0.05 
f l ux  
e+/ (m-2- 
str-sec-GeV) 
(0.3'2.7)E-3 
(0.2~0.5)E-2 
0.0252 0.008 
0.050 * 0.018 
0.161 * 0.035 
0.190.rO.094 
exposure 
cmh2 
-st r -sec 
4082 23 
463t 12 
480 t 13 
501 L 13 
557: 13 
588213 
377 
, OG 6.2-5 
servation are t i d y sing t e ta in t e ri t t 
col n in bl  1   ratio f in bl  1 to t  ber 
in (1) g1 s a correction to t e ex r  f ctor r ported in 
( ). T l  2 i s t e co puted ex sur  f ctors and f
ef r nce 1 i s a etailed i cussion of t e correcti ns f r 
geo agnetic t f solar odulation and br sstrahlung energy 
l . In (1) a ean ISM energy ( ), an equiv l nt in i th 
(  E ) and a pr pagation effi i ncy (p) are co puted f r each 
efl ction i terval in bl  1  f ct rs are also i n in 
t ble 2  
E e+ e / (e" + e-) ex sur  p bE fl x 
(Ge ) counts c A 2 (GeV e / (m ,~ . 
- tr-s str ) 
 1.6 :t 1 . 0. !: .  408 ~ 23 1. 1 . 3~2.7)E-3 
13.  .  -t 1 . . t 0.  463 t 12 1. . ( .2-: . ) -  
9. 30.6 !: 1 . 0.08.t 0. 480 t 13 .  . . : .
7. 2  :! 9. 0.06 x. 0. 501 .  13 0. 1.  0.  t 0.
6. 3 !: 8. 0. 1. 0. 557 ~ 13 0. 1.02 o. 1 ~ 0.
5. 14.  ! 7. O. 0 1. o. 05 588 !.  0. 0.  O.  :!: O. 094 
i re 2 sho  t e fl s as deter i ed in bl  2 along ith 
t e r sults of ot er o serv r , and t e fl s r dicted f r a 
si ple l aky box ( ith energy dependendt l akage) and 7 gm/cmA2 
of at er tr r d. A l ast squar s fit to t e fl s in table 
2 gi s a spectral i ex of - !0.6 and an i tegral flux (above 
5. ev) of ~.51!0. e+/mA2-str-sec- eV. T  chi-s for t e 
fit  2.  
Eig!,!c~ ~"­
Posi tron Flu>: s 
In 
r'-
(\J 
<
w 10 1 I-=-~ __ ~ 
* 
• ThI  '" ork x 
• ::0 
 Buf Ington et at. (3) ~ 
m FClfl l  e at. (
,"",leaky box 7 g /cm .... 2 
B~fg!:~ £~§ 
(1) ol , R. L. et al. (1984), J 6!E, 62 .
(2) Stephens, S  A (1981), roc 17th I RC ( aris) 1 282.
(3) Buffi ton, A et al. (1975),  J !~~, 66 .
(4) Faneslow, J  L. ( 969),  J !~~, 77 .


Observation of cosmic ray positrons from 5 to 25 GeV

Observation of cosmic ray positrons from 5 to 25 GeV

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