Balloon flight data on upper limit solar gamma ray spectrum from quiet su

Mc Kenzie, D.

Pelling, R. M.

Peterson, L. E.

Schwartz, D. A.

English

NASA Technical Reports Server

F' 
. 
UCSD-SP-66-6 
:kGgJst  18, 1966 
(Submitted t o  J G R )  
'b . 
THE UPPER LIMIT SOLAR GAMMA-RAY SPECTRUM TO 10 MEV 
BY 
Laurence E.  Pe te rson ,  Daniel  A. S c h w u t z ,  
R .  M. P e l l i n g  and David McKenzie 
Department of  Physics 
Un ive r s i ty  of  C a l i f o r n i a ,  San Diego 
La  Jol la ,  C a l i f o r n i a  
I n  t h i s  l e t t e r  we wish t o  r e p o r t  new upper l i m i t s  on solar y-rays 
obta ined  from a ba l loon  f l i g h t  s e r i e s  i n  February 1966. These r e s u l t s ,  when 
combined wi th  o t h e r  r e c e n t  work, form a composite upper l i m i t  y-ray spectrum 
from t h e  q u i e t  sun over  t h e  range 20 kev t o  10 Mev which i s  about a f a c t o r  
of 30 lower than  t h a t  p rev ious ly  a v a i l a b l e .  Since s e v e r a l  experimental  
t echniques  were used t o  cover t h i s  l a r g e  energy range ,  t he  r e s u l t s  i n  the  
20-200 kev range w i l l  be presented  f i r s t ,  followed by t h e  1-10 Mev work. 
Upper l i m i t s  i n  t h e  130-800 kev reg ion  a r e  a v a i l a b l e  from previous work 
( F r o s t ,  e t  a l ,  1966). I n  none of t h e  observa t ions  r epor t ed  here  w a s  t h e r e  
any i n d i c a t i o n  of  a s t a t i s t i c a l l y  s i g n i f i c a n t  s o l a r  f l u x .  
Observations i n  t h e  lower energy range were obta ined  wi th  t h e  pro to-  
type  ve r s ion  of an X-ray d e t e c t o r  o r i g i n a l l y  designed f o r  an OS0 s a t e l l i t e  
(Hicks,  Reid,  Pe terson ,  1965). 
10 cm a r e a  N a I  s c i n t i l l a t i o n  counter wi th  an  an t ico inc idence  co l l ima t ing  
This d e t e c t o r  c o n s i s t s  of a 3 mm t h i c k  x 
2 
s h i e l d  of C s I .  S ince t h e  inhe ren t  event  rate of t h i s  d e t e c t o r  i s  very. low,  
t h e  major con t r ibu t ion  t o  t h e  background comes from t h e  atmospheric and 
cosmic X-rays e n t e r i n g  t h e  0.15 s t e r a d i a n  forward ape r tu re .  The d e t e c t o r  
2 
4 
R TMX OR AD NUMBER) 
https://ntrs.nasa.gov/search.jsp?R=19670001548 2020-03-16T17:45:27+00:00Z
2 
was mounted i n  a b d l o o n  gondola w i t h  t h e  e l e v a t i o n  and azimuth servo-  
c o n t r o l l e d ,  u s ing  o p t i c a l  s o l a  sensors .  Events from t h e  128-channel pu lse-  
h e i g h t  ana lyze r ,  p l u s  r a t e  monitoring, se rvo  housekeeping and temperature 
data were a l l  t r ansmi t t ed  on a PCM/FM/FM system. 
t h e  modified servo  c o n t r o l ,  i s  i d e n t i c a l  t o  t h a t  used i n  previous observa- 
t i o n s  of cosmic X-ray sources  (Pe terson ,  Jacobson, P e l l i n g ,  1966) (Peterson 
and Jacobson, 1966). 
and data r educ t ion  procedures a re  a l s o  s i m i l a r  t o  t h a t  used by F ros t ,  e t  a l  
(1966) t o  o b t a i n  t h e  r e s u l t s  i n  the  130-800 kev range.  
r epor t ed  he re ,  t he  servo was programmed t o  t r a c k  t h e  sun f o r  40 minutes.  
The d e t e c t o r  w a s  then  poin ted  a t  t h e  same e l e v a t i o n  i n  r eve r se  azimuth 
d i r e c t i o n  f o r  20 minutes t o  ob ta in  a background measurement. Five minutes 
of each background measurement were devoted t o  c a l i b r a t i o n  w i t h  a B a  
source .  T h i s  sequence w a s  repeated each h m r .  
The appara tus ,  except f o r  
The genera l  d e t e c t o r  scheme, t e l eme t ry ,  servo system 
For the  observa t ions  
133 
The ba l loon  ca r ry ing  t h i s  instrument  w a s  launched a t  0445 MST on 
February 18, 1966 from Phoenix, Arizona, reached c e i l i n g  a l t i t u d e  of 3.5 
gn/cm2 at 0725 MST and apparent ly  f l o a t e d  l e v e l  u n t i l  f l i g h t  te rmina t ion  
a t  1555. 
t imer  i n i t i a t e d  the observ ing ,  background and c a l i b r a t i o n  sequence at 0723. 
The sun reached i t s  maximum e l e v a t i o n  of 45' a t  1210 MST. A 
Readouts of  t h e  azimuth and e l e v a t i o n  independent of the  servo loop 
Pulse-height  s p e c t r a  ob- v e r i f i e d  t h e  p o i n t i n g  and t r ack ing  ope ra t ions .  
t a i n e d  from a background measurement are sub t r ac t ed  from t h a t  ob ta ined  
dur ing  t h e  corresponding s o l a r  observa t ion .  
t h e  same e l e v a t i o n  ang le ,  c o r r e c t l y  account for dependence of t h e  background 
on z e n i t h  angle .  
These measurements, being at 
By c o r r e c t i n g  t h i s  d i f f e r e n c e  f o r  a r e a ,  e f f i c i e n c y ,  dead 
3 
t ime ,  and atmospheric absorp t ion ,  s t a t i s t i c a l l y  determined l i m i t s  on t h e  
s o l a r  f l u x  can be obta ined .  
used t o  o b t a i n  t h e  r e s u l t s  repor ted  he re .  
About 6.5 hours of t o t a l  observing time were 
The count ing r a t e s  and s t a t i s t i c a l  s i g n i f i c a n c e  of a l l  t h e  d a t a  used 
when observing t h e  sun a r e  ind ica t ed  i n  Table 1, as we l l  as t h e  d i f f e r e n c e  
a t  95% confidence l e v e l ,  cor rec ted  and ex t r apo la t ed  t o  zero depth .  
channel groupings have been chosen, c o n s i s t e n t  with the  energy r e s o l u t i o n  
and s p e c t r a l  s lope ,  t o  ob ta in  the lowest  p o s s i b l e  l i m i t s .  The r e s u l t s  a r e  
a l s o  p l o t t e d  i n  F igure  1 on a composite d i f f e r e n t i a l  f l u x  spectrum f o r  t h e  
q u i e t  sun. The h igher  energy d a t a  and t h e  r e s u l t s  of prev ious  work a r e  a l s o  
i n d i c a t e d .  Our upper l i m i t  spectrum over t h e  20 t o  200 kev range i s  con- 
s i d e r a b l y  lower than  those  obtained from t h e  OSO-I s a t e l l i t e  i n  March of 
1962, and j o i n s  smoothly onto  the r e s u l t s  i n  t h e  in te rmedia te  range r epor t ed  
by F r o s t ,  e t  a1 (1966) f o r  June 10, 1962. 
The 
Since d i r e c t i o n a l  d e t e c t o r s  which ope ra t e  a t  h igher  ene rg ie s  p re sen t  
a cons iderable  problem, t h e  r e s u l t s  i n  t h e  1-10 Mev range were obta ined  
us ing  a simple technique.  
an omnidi rec t iona l  geometry f a c t o r  of  67 cm 
w a s  flown a t  126,000 f e e t  during a nidht-day f l i g h t .  
ob ta ined  from t h e  d i u r n a l  v a r i a t i o n  of count ing r a t e s .  Considerable a t t e n -  
t i o n  t o  in s t rumen ta l  d e t a i l  i s  r equ i r ed  t o  o b t a i n  a spectrum l i m i t e d  only  
by t h e  count ing s t a t i s t i c s .  
t h i c k  p l a s t i c  s c i n t i l l a t i o n  s h i e l d ,  viewed by a second phototube and con- 
nec ted  i n  e l e c t r i c a l  an t ico inc idence .  This  e l imina te s  edge e f f e c t s  due t o  
cosmic r a y  p a r t i c l e s  and reduces t h e  count ing r a t e  i n  t h e  1-10 Mev range 
A 3" x 3" N a I  s c i n t i l l a t i o n  counter ,  which has  
2 and n e a r l y  an i s o t o p i c  response,  
The solar  f l u x  i s  then  
The d e t e c t o r  i s  surrounded wi th  a 2.5 em 
\ 
I .  
4 
t o  about 47 c /sec .  Each event  i s  coded wi th  a 128-channel pulse-height  
ana lyzer  onto a d i g i t a l  system, i n  a manner i d e n t i c a l  t o  t h a t  descr ibed  
p rev ious ly .  Gain c a l i b r a t i o n  i s  ob ta ined  by switcli ing,  tIirutig11 a krlowii 
r e s i s t a n c e  network, t o  t h e  200 kev t o  2 Mev range f o r  a b a t  5 minutes each 
hour .  This b r i n g s  t h e  0 .5  Mev a n n i h i l a t i o n  l i n e ,  a prominent f e a t u r e  of  
t h e  atmospheric 'y-ray spectrum, i n t o  t h e  range of  t h e  analyLer.  Tlle r e so lu -  
t i o n  of  t h e  d e t e c t o r  a t  t h i s  energy i s  14%. 
The p r i n c i p a l  r e s u l t s  a t  the h igher  ene rg ie s  t o  be repor ted  here  were 
a l s o  obt,ainetl February 18, 1966. 
1859 i"3T and e i l i n ~ r  was rea,chtil a t  2144 MST. T h i s  i- : i l loon riisc~ f l o a t t ~ l  l f v t l  
at 3.6 gm/cm2 u n t i l  t e rmina t ion  occurred a t  0940 MST. 
0300 t o  0500 were sub t r ac t ed  from t h e  spectrum between 07ClO and 0900, when 
t h e  s u n  w a s  a t  l e a s t  35 above the hor izon .  Since n o  l i i i c >  .,t i ,1 i f* ' rurC> wac 
observed, t he  pu l se  he ight  channels hpre have a l s o  been cmbirictl t o  L'orm 
appropr i a t e  energy groupings.  Table I i n d i c a t e s  t h e  r a t e  over t hese  i n t e r -  
v a l s ,  t he  s t a t i s t i c a l  e r r o r ,  and t h e  upper l i m i t  y-ray Ylux a t  :t 95% con- 
f idence  l e v e l .  This f l u x  i s  obtained once aga in  by co r rec t ing  the  count ing 
r a t e  d i f f e r e n c e  f o r  a r e a ,  channel width and atmospheric depth .  No co r rec t ions  
f o r  e f f i c i e n c y  have been appl ied he re ,  a l lowing a d i r e c t  comparison w i t h  
p rev ious ly  r epor t ed  r e s u l t s  i n  the 1-10 Mev range.  These r e s u l t s  a r e  a l s o  
shown i n  Figure 1, as w e l l  as r e s u l t s  ob ta ined  i n  an e a r l i e r  observa t ion  
w i t h  t h e  same appara tus  on June 27, 1965. The cons iderable  rerliiction i n  
upper l i m i t s  on t h e  most r e c e n t  f l i g h t  occurred by reducing systelnat,ic un- 
r e r t a i n t i e s  and improving d a t a  a c q u i s i t i o n  procedures .  
Launch occurred the  previous pvening :it 
Data obta ined  from 
0 
5 
e 
H All these data form a complete set of upper limits on the quiet 
sun y-ray flux over the 20 kev to 10 Mev range. The usual indicators of 
s o l a s  activity, flares, sunspot number, radio emission, etc. showed none 
of the variations or enhancements which characterize an active sun. (Pre- 
liminary report of Sola r  Activity, February 2 5 ,  1966), (Compilations of 
Solar-Geophysical Data, March 1966, June 1966). These new limits are 
statistically determined at about 3% of the atmospheric y-ray flux at 
3.3 @/cm 
are about one and a half ordersof magnitude lower than those ljreviouslLr 
available. Atmospneric absorption limits balloon observations to energies 
above about 20 kev. 
2 
and h = 40' magnetic. As indicated in the figure these limits 
Also shown in the figure are rocket data obtained at loder energies 
by the Livermore group in June l$b5 (Chodil, 1965) fitted to an optically 
thin riot gas at 4.5 x 1(j6 c, K. 
"2 - 8 8. region", due to the solar corona or hot re ions t l t  11 'F active 
centers nave been made by Friechiml1 A n 1  riis Associates ( K r e p l i n .  1) 2 ) .  
Since the observations indicate that these fluxes are ni : ~ l y  variable in 
time, the concept of 3 "quiet sun" may not be applicable to this spectral 
region. During solar flares, fluxes many times auovit. tnese limits riive 
been occasionally observed, having energies up to several iiundred kev 
(AAS-NASA Symposium, 1964) 
E ~ C I  ,ive measurement? oi t l i eze  X-ra. S .  e 
Dolan and Fazio (1965) have recently reviewed the observations and 
theory regarding the solar y-ray spectrum. 
of solar processes, the rnost probable y-ray emission process is due to 
cosmic rays interacting near the solar surface and producing secondary 
Based on present understanding 
6 
y-ray albedo. 
gested as an important source, have been shown to contribute neglectable 
fluxes, most likely even during solar flares. However, y-ray emission 
Thermonuclear reactions in the solar atmosphere, once sug- 
will occur on the quiet sun due to the naturally occurring radioactive 
isotopes. These y-ray lines can be predicted, based on the abundances 
quoted by Goldberg, Muller, and Aller (1960) for the chromosphere. 
assumes sufficient mixing takes place so that the upper 30 gm/cm2 of the 
One 
photosphere has the same element ratios as the chromosphere. The pre- 
dieted fluxes, considerably below the present upper limits, are 0.48 x 10 
photons/cm2-sec at 1.48 Mev due to K 4 0 ,  and 0.21 x 
-8 
photons/cm2-sec due 
228 to Th . 
Cosmic-ray produced y-rays can be estimated from measurements on 
terrestrial albedo. This intensity,shown in the Figure, is obtained by 
L dividing the background at 3.6 gm/cm 
to zero depth, multiplying by 3 to correct from h = 40' to h = 0, and 
by a factor of 1.5 which extrapolates 
. As indicated by the .25 x 10-3 
%n 
accounting for the solid angle ratio 
dashed line, this albedo must join the extension of the coronal spectrum 
in the 8 - 20 kev range. 
cosmic rays is indeed incident on the solar surface. 
condition is not obtained due to solar and interplanetary magnetic fields; 
therefore the estimated albedo is an upper bound. For example, if the 
surface dipole field is 1 gauss, the resultant magnetic cutoff for protons 
traveling in the meridian plane and arriving at the equator would be 
This assumes the full intensity of galactic 
Most likely this 
solar equator. 
a similar effect. 
Random fields in the interplanetary medium could produce 
7 
Although these new upper limits put considerable constraints on non- 
thermal process, and are at a level much below previous measurements, 
further reduction is needed. In particular, it would be desirable to 
place upper limits at the level of the "estimated albedo". This requires 
only  about a factor of 30 increase in sensitivity over the 20 - 100 kev 
range. The techniques to accomplish this are under development now, 
however at the higher energies nearly a factor of lo3 is needed. 
methods of achieving such low backgrounds and high collimations are not 
yet even in a conceptual stage. 
Acknowledments 
Here the 
The authors are indebted to Rod Jerde who designed and constructed 
the balloon gondola and servo system. Paul Brissenden and James Matteson 
manned the downwind telemetry station which made the multiple flight ar- 
rangement possible, and assisted in the flight preparations. The balloon 
operations were provided through the NCAR facilities at Palestine, Texas. 
This research is supported under NASA grant NsG-318 and contract NAS 5-3177. 
a 
References 
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s p e c t r a  from Scorpius  (SCO-XR-1) and t h e  sun observed above t h e  atmosphere, 
Phys. Rev. L e t t e r s ,  15, 605-607, 1965. 
Dolan, J. F. and G .  G. Faz io ,  The gamma-ray spectrum of the  sun, E. Geophys., 
3, 319-343, 1965. 
F r o s t ,  K .  K . ,  E .  D .  Rothe, and L.  E .  Pe te rson ,  A search f o r  t he  q u i e t  time 
solar  gamma rays  from ba l loon  a l t i t u d e s ,  - J. Geophys. - Res.,  1966 ( I n  p r e s s )  
Goldberg, L . ,  E .  A .  M a l e r ,  and L. H .  A l l e r ,  The abundances of t h e  elements 
i n  t h e  s o l a r  atmosphere, Astrophys. - J . ,  (Sup. 45) 5, 1-138, 1960. 
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Orb i t ing  S o l a r  Observatory,  IEEE Trans .  on NUC. S c i . ,  1 2 ,  54-65, 1965. -- -- - 
Krepl in ,  R .  W . ,  T .  A .  Chubb, and H.  Friedman, X-ray and Lyman-alpha emission 
from t h e  sun as measured from the NRL SR-1 s a t e l l i t e ,  - J .  Geophys. Res., 67, 
2231-2253, 1962. 
Lincoln,  J.  V i r g i n i a ,  E d i t o r ,  Prel iminary Report  -- of Solar A c t i v i t y ,  Environ- 
mental  Science Serv ices  Adminis t ra t ion ,  February,  1966. 
Lincoln,  J. V i r g i n i a ,  E d i t o r ,  Compilations -- of  Solar-Geophysical - Data, Environ- 
mental  Science Se rv ices  Adminis t ra t ion,  March and June,  1966. 
Peterson ,  L. E . ,  A .  S .  Jacobson, and R .  M. P e l l i n g ,  Spectrum of Crab Nebula 
X-rays t o  120 kev, Phys. Rev. L e t t e r s ,  16, 142-144, 1966. 
Peterson ,  L .  E . ,  and A .  S .  Jacobson, The spectrum o f  SCO-XR-1 t o  50-kev, 
Astrophys. J . ,  1966 ( I n  p r e s s ) .  
Figure 1 
Measurements and estimates on the quiet sun y-ray spectrum. The present, 
upper limits from a series of balloon observations on February 18, 1966 
are about a factor of thirty below the previous observations from the 
OSO-I. The hot solar corona produces a steep frez-free emission spectrum 
which must join smoothly onto the estimated albedo flux due to cosmic-ray 
interactions in the solar surface. 
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The Upper Limit Solar Gamma-ray Spectrum to 10 Mev

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