A balloon flight of the University of Chicago electron telescope was performed. Making use of the east-west asymmetry in the geomagnetic cut off rigidity, the cosmic ray positrons and negatrons were separated over the range 10 GeV to 20 GeV. The positron to electron ratio, e+/(e++e-), was measured to be 17% + or - 5%, significantly higher than the ratio measured in the 1 GeV to 10 GeV range by other experiments. This increase appears to suggest that either a primary component of positrons become significant above 10 GeV, or that the spectrum of primary negatrons decreases above 10 GeV more sharply than that of secondary positrons

Mueller, D.

Tang, J.

English

NASA Technical Reports Server

Diet r ich  Miller' and Jonathan Tang 
Enrico Fermi I n s t i t u t e ,  University of Chicago 
I l l i n o i s  60637 USA. 
ABSTRACT A b a l l o o n  f l i g h t  of t h e  U n i v e r s i t y  of  Ch icago  
e l e c t r o n  t e l e s c o p e  has  been performed i n  Hawaii i n  A p r i l  1984. 
Making use of the east-west asymmetry i n  t h e  geomagnetic cu t -of f  
r i g i d i t y  , we have s u c c e s s f u l l y  separated the  cosmic-ray posi t rons 
and negatrons over the range  10-20 GeV. The p o s i t r o n  t o  e l e c t r o n  
r a t i o ,  e+ / ( e++e- ) ,  was measured t o  be 17%25%, s ign i f i can t ly  higher 
than the  r a t i o  measured i n  the  1-10 GeV range by other  experiments .  
Th i s  i n c r e a s e  appears t o  suggest t h a t  e i t h e r  a  primary component of 
positrons become s ign i f i can t  above 10 GeV, or  t h a t  t h e  spectrum of 
primary nega t rons  decreases  above 10 GeV more sharply than t h a t  of 
secondary positrons. 
INTRODUCTION Cosmic r ay  e l e c t r o n s  consis t  of both posi t rons and 
negatrons. Present measurements of t h e  combined p o s i t r o n  and n e g a t r o n  
f l u x  cover  energies up t o  almost 1000 GeV, while data  on the  charge com- 
posi t ion beyond 10 GeV a r e  very  scarce .  The combined energy spectrum 
exhib i t s  a  cha rac t e r i s t i c  steepening around 30 GeV, which i s  commonly at-  
t r ibu ted  t o  r ad i a t ive  energy losses  i n  i n t e r s t e l l a r  space. I f  interpreted 
i n  t h e  context of the leaky box ~ o d e l ,  the steepening i s  consis tent  with 
a  ga l ac t i c  containment time of 10 years, i n  close agreement with r e s u l t s  
based on measurements of the ' O B ~  abundance. However, these in te rpre ta -  
t ions  of the e lec t ron  spectrum a r e  usually made under the  assumption t h a t  
t h e  source spectrum of e lec t rons  follows t h a t  of nuclear cosmic rays and 
t h a t  the e lec t ron  f l u x  i s  dominated by primary negatrons. This assumption 
must be v e r i f i e d  by s e p a r a t e  measurements of posi t rons and negatrons: 
Both posi t rons and negatrons a r e  produced i n  about equa l  p r o p o r t i o n s  a s  
secondary products of i n t e r ac t ions  of cosmic ray nuclei  with i n t e r s t e l l a r  
gas. The source spectrum of these secondary p a r t i c l e s  i s  d i r ec t ly  re la ted  
t o  t h e  spectrum of p a r e n t  p a r t i c l e s  (mostly protons) and can be calcu- L 
l a t e d  w i t h  good accuracy  f o r  v a r i o u s  p r o p a g a t i o n  m o d e l s  ( O r t h  & 
Buff ington ,  1976). Any s igni f icant  excess of negatrons, however, must be 
a t t r i bu ted  t o  primary acce lera t ion  i n  cosmic ray sources.  Below 10 GeV, 
such primary negatroas have been found t o  be the dominant component. 7 
Most of the ex i s t i ng  data  on cosmic ray 
p o s i t r o n s  come f rom b a l l o o n  borne magnet NORTH 
spectrometers (Fanselaw e t  a1 1 %9 ,  Buff ington 
e l  a1 1975, Daugherty e t  a 1  19751, but both 
momentum r e s o l u t i o n  and coun t ing  s t a t i s t i c s  
have l i m i t e d  t h e  acces s ib l e  energy range. An 
,I, 
a l t e r n a t e  opportunity t o  separate  pos i t ive  and 
n e g a t i v e  p a r t i c l e s  i s  provided by the e a r t h ' s  
magnetic f i e l d .  The cu t -of f  r i g i d i t y  v a r i e s  
with the geographic locat ion,  the  d i rec t ion  of 
incidence of t h e  p a r t i c l e  and w i t h  the  par- 
t i c l e  c h a r g e .  The c u t - o f f  r i g i d i t i e s  f o r  
- 
negatrons a t  Hawaii a r e  shown i n  f i gu re  1. Fig.1 View of Sky, Hawaii 
=~upported, i n  part, by NASA grant NSG-7464. contwo of negatron cutoff rigidities 
Also Department o f  Physics. 
378 
* AN OBSERVATION OF COSMIC RAY POSITRONS FROM 10-20 GEV 
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https://ntrs.nasa.gov/search.jsp?R=19850025792 2020-03-20T17:58:52+00:00Z
For posi t rons,  east  and west exchange ro l e s .  Theref o r e ,  t h e r e  e x i s t s  a  
range of energies  and d i rec t ions  f o r  which, for  a  given geographic loca- 
t ion ,  the e a r t h ' s  f i e l d  t ransmi ts  on ly  p a r t i c l e s  of one p o l a r i t y ,  The 
u t i l i z a t i o n  o f  t h i s  e f f e c t  was p i o n e e r e d  by Danie l  and Stephens  
(1965,1967). However, the f i r s t  measurements (Anand e t  a l ,  1%9; Agrinier  
e l  a l ,  1969) were seve re ly  l i m i t e d  due t o  i n su f f i c i en t  s t a t i s t i c s  and 
systematic uncertaint ies .  
I n  April  1984, we have performed a balloon f l i g h t  from Hawaii using 
our e lec t ron  telescope t o  separate negatrons and p o s i t r o n s  i n  t h e  range  
10-20 GeV. The f l i g h t  had a  duration of 11 hours a t  a  f l o a t  a l t i t u d e  cor- 
responding  t o  4 gm/cm2 r e s i d u a l  a tmosphe re  i n  t h e  v e r t i c a l .  The 
following descr ibes  our measurement. 
INSTRUMENTATION This instrument has been d e s c r i b e d  p r e v i o u s l y  i n  
d e t a i l  (Tang, 1984).  Here we ju s t  summarize the main fea tures :  The in- 
strument i s  an e l ec t ron ic  counter te lescope w i t h  a  geometr ic  f a c t o r  of 2  
about  O . l m  sterad. For each p a r t i c l e  t raversing t h e  detector ,  the fol-  
lowing q u a n t i t i e s  a r e  measured: (1) t he  charge number lzl with a  p l a s t i c  
s  c i n t  i l l a t o r ,  (2) t he  t r a j ec to ry  through the instrument with a  mu1 t i w i r e  
proportional chamber hodoscope, ( 3 )  t h e  d i r e c t i o n  of t r a v e r s a l  w i t h  a  
t ime-o f - f l i gh t  measurement, ( 4 )  t h e  t r a n s i t i o n  r a d i a t i o n  emi t ted  by 
e lec t rons  (but not protons) i n  a  6-layer t r a n s i t i o n  r a d i a t i o n  d e t e c t o r ,  
and  ( 5 )  t h e  e l e c t r o m a g n e t i c  cascades  produced i n  a  9  l a y e r  lead-  
s c i n t i l l a t o r  shower d e t e c t o r  of 18.5 r a d i a t i o n  l e n g t h  depth. T h e s e  
cascades a r e  c h a r a c t e r i s t i c  fo r  e lectrons and measure the  e lec t ron  energy 
with &E/E ~ 8 % .  
During t h e  bal loon f l i g h t ,  the instrument has been suspended with 
i t s  a x i s  inc l ined  by 30 degrees against the  ve r t i ca l .  The o r i e n t a t i o n  of 
t h e  gondola was measured w i t h  magnetometers, and, through the use of a  
comaandable torque motor, the instrument could  be o r i e n t e d  towards t h e  
west ( t o  obse rve  pos i t rons)  o r  towards the eas t  ( f o r  a  control  measure- 
ment of negatrons). To determine the cut-of f  r igidi t ies(shown i n  f i g .  1 ), 
we use t h e  predict ions of Shea and Smart (19741, and we note t h a t  cosmic 
ray measurements of Jordan and Meyer (1983) have been i n  agreement w i t h  
these predictions.  
ANALYSIS & RESULTS Electrons a r e  selected from t h e  d a t a  s e t  ac-  
c o r d i n g  t o  p r o c e d u r e s  d e s c r i b e d  i n  d e t a i l  i n  o u r  e a r l i e r  work  
(Tang,1984). Due t o  t h e  combinat ion of a deep shower d e t e c t o r  w i t h  a  
t r a n s i t i o n  r a d i a t i o n  d e t e c t o r ,  our  instrume t i s  capable of r e j ec t ing  8 protons with the required r e j ec t ion  power of 10 . The r e s i d u a l  p r o t o n  
contaminat ion  t o  our r e s u l t s  i s  indicated i n  t a b l e  1. After  applying t h e  
appropriate  cor rec t ions  f o r  i ne f f i c i enc i e s  of i n d i v i d u a l  d e t e c t o r  e l e -  
m e n t s ,  f o r  t h e  a r e a - e x p o s u r e  t i m e ,  and  f o r  energy l o s s e s  i n  t h e  
res idua l  atmosphere above the detector ,  we o b t a i n  t h e  e l e c t r o n  f l u x e s ,  
grouped according t o  d i rec t ions  of incidence from EAST, and from WEST. 
The measured EAST and WEST e l ec t ron  spec t ra  a r e  shown i n  Figure 2. 
They exhib i t  a .  c l e a r  eastlwest asymmetry between 10 t o  20 GeV while show- 
ing the  same f l u x  a t  o ther  energies. Cutoffs a t  11 GeV ( e a s t )  and 22 GeV 
(west ) a g r e e  well  with negatron cut-of f  ca lcu la t ions ,  which use codes by 
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spectrum can be analyzed a s  a 
composite of ( i )  atmospheric . 2 -  
s e c o n d a r y  e l e c t r o n s ,  ( i i )  
secondary e l e c t r o n s  t h a t  ap- r 
p e a r  a s  r e e n t r a n t  a l b e d o  
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positrons. - 
02 - 1' 
n 
Such an ana lys is  f o r  the S 
.01 = WEST d a t a  i s  shown i n  Figure 
2. The curve f o r  a tmosphe r i c  
.005 - 
s e c o n d a r i e s  i s  d e r i v e d  from 
in te rpola t ing  c a l c u l a t i o n s  b 1 Beuermann (1971) f o r  7 gm/cm .002., u t t a r  
of atmosphere. The r e e n t r a n t  5 10 GeV 50 5 10 GEV 50 
a lbedo par  t i c l e s  a r e  expected Fig.2 Differential Energy Spectra of Electrons. Hawaii '84 
t o  d i  s appea r  abov e c u t -  o f  f  The solid curve (WEST) is the sum of the dotted curves. 
ln pigure 2 ,  we The dashed curve is  a hand fit to the EAST spectrum. Reentrant albedo data point: V Daniel k Stephens '67 
e s t i m a t e d  t h e i r  f l u x e s  such  
t h a t  t h e  d i r e c t  atmospheric and the  reentrant  eecondarier add up t o  our 
d a t a  below 10 GeV. T h i s  procedure  i s  c o n s i s t e n t  w i t h  t h e  f l u x  o f  
r e e n t r a n t  a lbedo  e l e c t r o n s  a t  4 GeV measured by Danie l  and Stephens 
( 1 9 6 7 ) .  
The curve of g a l a c t i c  nega t rons  i s  obta ined  by folding our previously 
measured e l ec t ron  spectrum wi th  c a l c u l a t i o n s  of geomagnet ic  cu t  o f f  s .  
Clearly, a s ign i f icant  amount of posi t rons i s  required t o  account f o r  the 
f luxes  from 10 t o  20 GeV. 
To d e r i v e  t h e  pos i t ive  f r a c t i o n  of cosmic ray electrons,  the WEST 
data a r e  divided i n t o  2 energy in t e rva l s  shown i n  T a b l e  1. N( to ta1)  i s  
t h e  number of s e l e c t e d  e v e n t s  and  proton) is the  estimated number of 
residual  proton background. ~ ( a t m o s )  i s  t h e  number of a tmospher ic  and 
r e e n t r a n t  a lbedo  even t s  estimated by extrapolat ing data  below 10 GeV t o  
higher energies. The net  number of positrons, N(+) , is  obtained by sub- 
t r ac t ing  N(proton) and ~ ( a t m o s )  from N( t o t a l ) .  We obta in  N( sum), the sum 
of cosmic posi t rons and negatrons, using the  EAST d a t a  and ou r  p rev ious  - 
measurement ( ~ a n g  1984). Final ly ,  a fac tor  f  co r r ec t s  f o r  small e f f e c t s  
due t o  the f i n i t e  energy reso lu t ion  of the detector  and t h e  v a r i a t i o n  of 
cut-of f  r i g i d i t i e s  over extended regions of the sky. The pos i t ive  f rac-  
t ion ,  e+/(e++e-), i s  then the r a t i o  of N(+) and  sum), mul t ip l ied  by f .  
Table 1 
N(sum) f + E ( G ~ V )  N(tota1) N ( D ~ o ~ o ~ )  ~ ( a t m o s )  N(+) e /(e++e-) 
10-14.4 67+8.2 9.352.3 22.25.8 35.8+10 248+_30 1.05+.05 .151+.048 
14.4-20 34z5.8 3 .39 .8  8.623.4 22.127. 97213 0.902.10 .204+.072 
DIS CU S SION + + O u r  measurements of e / ( e  +e-1 i n  cosmic-ray a r e  shown i n  
F i g u r e  3 .  The e r r o r  l i m i t s  i n c l u d e  s t a t i s t i c a l  and  s y s t e m a t i c  - 
u n c e r t a i n t i e s .  We compare our  data with measurements a t  lower energies 
and with some ca lcu la t ions  of propagation models using pa th lengths  which 
agree with B/C  r a t i o s  (Protheroe, 1982). The ca lcu la t ions  have shown t h a t  
Shea and S art (197 ). Each 
spectru  can be analyzed as a 
co posite of (i) at ospheric 
secondary electrons, (ii) 
secondary electrons that a
pear as reentrant albedo 
ticles, (iii)  ~ 5 
.1 
.2 
310 
East 
:~!~~~~::: and (iv) galactic ~02 ~V\ 
West 
Such an analysis for the '1 atmosph.' !" 
01 secondary \ • + ..... EST data is sho n in Figure . 11 ~ 
2. The curve for at ospheric ;\,\". \, 
secondaries is derived from .005 
r •• ntrant \\ interpolating calculations b~ albedo \\ 
euer a  (1971) for 7 g /  02~~wu~__ -L __ L-~_~~~~L-_\_\~~~~ 
of at os r . The reentrant 5 10 CeV 50 5 10 
albedo p ticles are expected 
to disappear abo e c o f 
rigidities. In Figure 2, e 
esti ated their fluxes such 
50 
CeV 
Fig.2 Dif erential Energy Spectra of Electrons, Hawai  '84 
The solid curve ( EST) is the sum of the dotted curves. 
The dashed curve is a hand fit to the EAST spectru . 
Reentrant albedo data point: \l Daniel &c Stephens '67 
that the irect at ospheric and the reentrant secondaries add up to our 
data belo  10 e . his rocedure is c istent ith the flux f 
reentrant albedo electrons at 4 e  easured by aniel and Stephens 
(1 ). 
The curve of alactic negatrons is obtained by folding our previously 
easured electron spectrum. ith calculations of geo agnetic c s. 
learl , a significant a ount of positrons is required to account for the 
f1 xe s fro  10 to 20 e .
To derive the p sitive fraction of cos ic ray electr s, the ES  
data are divided into 2 energy intervals sho n in able 1. l) is 
the nu ber of selected events and N(proto ) is the esti ated nu ber of
residual proton bac r . N(atmos) is the nu ber of at ospheric and 
ree trant albedo events esti ated by extrapolating data belo  10 Ge  to 
higher ener i . The net nu ber of p sitr , (+) , is obtained b  s
tracting ( r t ) and N(atmos) fro  1  e btain su ), the s
of cos ic positrons and negatr s, sing the E S  da ta and our revious 
easure ent (Tang 1984). Fina11 , a factor f corrects for sma11 effects 
due to the finite energy resolution of the detector and the riation of
c ff rigidi ties over extende  regions of the sk . The sitive fra -
tion, e /(e , is then the ratio o (+) and N(sum , ltiplied b  f. 
E(GeV) (tota1) (proton) 
~14 4 67.±  9.3.± .  
1 -20 34.±S 3.3.±Q.8 
able 1 
N(atmos) (+) 
22.~S 8 3S ~
8. ~ 4 22.1.±7. 
(su ) 
248.±30 
97.± 3 
f 
1. S.±.05 
O .±.10 
+ +-
e /  e  
.1 .±.048 
• .±  0  2 
+1 ( + -) . . ISCUSSI  ur easure ents o e  e 1  cos 1  re sho n in 
igure . he rror li its include tatistical and syste atic 
 e co pare our data ith easure ents at lo er energies 
and ith so e calculations of ropagation odels sing ath1 ths ich 
agree ith c ratios ( rotheroe, 1982). he calculations have sho n that 
• 
t h e  p o s i t r o n  f l u x  below 10 GeV i s  c o n s i s t e n t  w i t h  a n  e n t i r e l y  ( > 9 7 % )  
s e c o n d a r y  o r i g i n ,  due t o  i n t e r a c t i o n s  of nuc le i  (mostly protons)  i n  t h e  
i n t e r s t e l l a r  gas. Our r e s u l t s  show an inc rease  of t h e  p o s i t i v e  f r a c t i o n  
a b o v e  1 0  GeV and a r e  s i g n i f i c a n t l y  higher than t h e  model c a l c u l a t i o n s .  
+ There a r e  two p o s s i b i l i t i e s  t o  u n d e r s t a n d  t h i s  i n c r e a s e  of 
e + / ( e  + e  ) : ( 1  ) a pr imary component of pos i t rons  becomes s i g n i f i c a n t  
above 10 GeV; o r  ( 2 )  t h e  f l u x  of primary e l e c t r o n s  (assumed t o  be a l m o s t  
e n t i r e l y  n e g a t r o n s )  d e c r e a s e s  above 1 0  GeV more r a p i d l y  than t h a t  of 
secondary  p o s i  txons .  The f i r s t  
p o s s i b i l i t y  may not  be very l i k e l y ,  .5 
while the  l a t t e r  i s  corroborated by 
t h e  f a c t  t h a t  t h e  t o t a l  e l e c t r o n  
spectrum begins t o  bend ovfr a$ th_e .2 
same energy reg ion  where e  / ( e  +e ) 
i n c r e a s e s .  S u c h  a  d e c r e a s e  o f  .1 
p r i m a r y  n e g a  t r o n s  a b o v e  1 0  GeV 
c o u l d ,  f o r  i n s t a n c e ,  h a v e  i t s  .05 
o r i g i n  i n  t h e  a c c e l e r a t i o n  reg ion  
i f  s y n c h r o t r o n  l o s s e s  i n  t h a t  
r e g i o n  1 i m i t  t h e  e l e c t r o n  energ ies  
a t t a i n a b l e  dur ing t h e  a c c e l e r a t i o n  '02 
t ime .  O r  it may be t h a t  sources  of 
primary e l e c t r o n s  a r e  s i g n i f i c a n t l y  0.1 I. GeV lo .  50. 
f u r t h e r  away t h a n  those  of secon- Fig.3 Positive Fraction of Cosmic Ray Electrons 
d a r y  p o s i t r o n s ,  l e a d i n g  t o  data: *This Work. v Buffinston '75. Oaugherty '75. 
c o m p a r a t i v e l y  l a rger  energy losses  0 Fonsalow ' 6 9 . 0 ~ ~ r i n i m r  '69. cunms: Prothmroe '82. 
e n  route .  
ACKNOWLEDGEMENT We acknowledge t h e  a s s i s t a n c e  of E. Drag and t h e  
s u b s t a n t i a l  con t r ibu t ions  of R. Kroeger towards t h e  s u c c e s s  of t h i s  ex- 
p e r i m e n t ,  and  we a r e  i n d e b t e d  t o  t h e  crew of t h e  Nat ional  S c i e n t i f i c  
Balloon F a c i l i t y  and t h e  U. S. Navy fo r  e x c e l l e n t  ba l loon  f l i g h t  and  sup- 
p o r t  o p e r a t i o n s .  We a r e  a l s o  g r a t e f u l  fo r  t h e  h e l p  of D r .  Peter  Meyer i n  
shar ing  t r a n s p o r t a t i o n  and accommodation of our equipment. 
Reference s  
Agr in ie r ,  e t  a l .  L e t t e r e  a1  Nuovo Cimento, s e r i e  I, 1 53 (1968). 
Anand, K. C., Daniel R.R., and Stephens S.A., 11th  ICRC Budapest (1969). 
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An observation of cosmic ray positrons from 10-20 GeV

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