The structure of the CGRO satellite is irradiated by cosmic rays and trapped particles fromradiation belts. These incident particles produce radioactive nuclei in nuclear reactions with the satellite structure. Most of the radiation dose can be attributed to the passages through the South Atlantic Anomaly. The incident particle flux on the COMPTEL instrument is estimated from the event rate of a plastic scintillation detector. This event rate is modeled with a Neural Network simulation. The increase of the event rate during SAA passages is taken as a measure for the amount of induced radioactivity. A Neural Network Model is used to derive the buildup ofradioactive nuclei in the instrument over the first five years of the mission. Measurements of the internal 22Na - and 24Na-activity are used to estimate the proton flux in the SAA. The result is consistent with earlier measurements and models

Diehl, R

McConnell, Mark L

Oberlack, U

Ryan, James M.

VanDijk, R

Varendorff, M

Weidenspointer, G

Breton

UNH Scholars' Repository

University of New HampshireUniversity of New Hampshire Scholars' RepositorySpace Science Center Institute for the Study of Earth, Oceans, and Space(EOS)1997A time dependent model for the activation ofCOMPTELM VarendorffMax-Planck-Institut für extraterrestrische PhysikU OberlackMax-Planck-Institut für extraterrestrische PhysikG WeidenspointerMax-Planck-Institut für extraterrestrische PhysikR DiehlMax-Planck-Institut für extraterrestricheR VanDijkESTECSee next page for additional authorsFollow this and additional works at: https://scholars.unh.edu/sscPart of the Astrophysics and Astronomy CommonsThis Conference Proceeding is brought to you for free and open access by the Institute for the Study of Earth, Oceans, and Space (EOS) at University ofNew Hampshire Scholars' Repository. It has been accepted for inclusion in Space Science Center by an authorized administrator of University of NewHampshire Scholars' Repository. For more information, please contact nicole.hentz@unh.edu.Recommended CitationA time dependent model for the activation of COMPTEL Varendorff, Martin and Oberlack, Uwe and Weidenspointner, Georg andDiehl, Roland and van Dijk, Rob and McConnell, Mark and Ryan, James, AIP Conference Proceedings, 410, 1577-1581 (1997),DOI:http://dx.doi.org/10.1063/1.54049AuthorsM Varendorff, U Oberlack, G Weidenspointer, R Diehl, R VanDijk, Mark L. McConnell, and James M. RyanThis conference proceeding is available at University of New Hampshire Scholars' Repository: https://scholars.unh.edu/ssc/163A time dependent model for the activation of COMPTELMartin Varendorff, Uwe Oberlack, Georg Weidenspointner, Roland Diehl, Rob vanDijk, Mark McConnell, and James Ryan  Citation: AIP Conference Proceedings 410, 1577 (1997); doi: 10.1063/1.54049 View online: http://dx.doi.org/10.1063/1.54049 View Table of Contents:http://scitation.aip.org/content/aip/proceeding/aipcp/410?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Great science observatories in the space station era and OWL efforts in Japan AIP Conf. Proc. 433, 117 (1998); 10.1063/1.56105  Galactic diffuse γ-ray emission at TeV energies and the ultra-high energy cosmicrays AIP Conf. Proc. 410, 1203 (1997); 10.1063/1.54102  Modelling cosmic rays and gamma rays in the Galaxy AIP Conf. Proc. 410, 1162 (1997); 10.1063/1.54096  Contribution of passive materials to the background lines of the spectrometer ofINTEGRAL (SPI) AIP Conf. Proc. 410, 1559 (1997); 10.1063/1.54047  Diffuse galactic continuum radiation AIP Conf. Proc. 410, 192 (1997); 10.1063/1.54029   This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to  IP:  132.177.229.79 On: Mon, 24 Nov 2014 16:49:54A t i m e  d e p e n d e n t  M o d e l  f o r  t h e  a c t i v a t i o n  o f  C O M P T E L  M a r t i n  V a r e n d o r t P ,  U w e  O b e r l a c k * ,  G e o r g  W e i d e n s p o i n t n e r * ,  R o l a n d  D i e h l * ,  R o b  v a n  D i j k  t ,  M a r k  M c C o n n e l l  tt a n d  J a m e s  R y a n  tt * M a x - P l a n c k - I n s t i t u t  f i i r  extraterrestrische Physik, D-857~,0 Garching, G e r m a n y  t E S T E C ,  Space Science D e p a r t m e n t ,  Keplerlaan 1, NL-2201 A Z  Noordwijk, N e t h e r l a n d s  t t  University o f  N e w  Hampshire, Morse H a l l ,  Durham, N. H., 0382.~, USA A b s t r a c t .  T h e  s t r u c t u r e  o f  t h e  C G R O  s a t e l l i t e  i s  i r r a d i a t e d  b y  c o s m i c  r a y s  a n d  t r a p p e d  p a r t i c l e s  f r o m  r a d i a t i o n  b e l t s .  T h e s e  i n c i d e n t  p a r t i c l e s  p r o d u c e  r a d i o a c -  t i v e  n u c l e i  i n  n u c l e a r  r e a c t i o n s  w i t h  t h e  s a t e l l i t e  s t r u c t u r e .  M o s t  o f  t h e  r a d i a t i o n  d o s e  c a n  b e  a t t r i b u t e d  t o  t h e  p a s s a g e s  t h r o u g h  t h e  S o u t h  A t l a n t i c  A n o m a l y .  T h e  i n c i d e n t  p a r t i c l e  f l u x  o n  t h e  C O M P T E L  i n s t r u m e n t  i s  e s t i m a t e d  f r o m  t h e  e v e n t  r a t e  o f  a p l a s t i c  s c i n t i l l a t i o n  d e t e c t o r .  T h i s  e v e n t  r a t e  i s  m o d e l e d  w i t h  a N e u r a l  N e t w o r k  s i m u l a t i o n .  T h e  i n c r e a s e  o f  t h e  e v e n t  r a t e  d u r i n g  S A A  p a s s a g e s  i s  t a k e n  a s  a m e a s u r e  f o r  t h e  a m o u n t  o f  i n d u c e d  r a d i o a c t i v i t y .  A N e u r a l  N e t w o r k  M o d e l  i s  u s e d  t o  d e r i v e  . t h e  b u i l d u p  o f  r a d i o a c t i v e  n u c l e i  i n  t h e  i n s t r u m e n t  o v e r  t h e  f i r s t  f i v e  y e a r s  o f  t h e  m i s s i o n .  M e a s u r e m e n t s  o f  t h e  i n t e r n a l  22Na- a n d  2 4 N a - a c t i v i t y  a r e  u s e d  t o  e s t i m a t e  t h e  p r o t o n  f l u x  i n  t h e  S A A .  T h e  r e s u l t  i s  c o n s i s t e n t  w i t h  e a r l i e r  m e a s u r e m e n t s  a n d  m o d e l s .  I N T R O D U C T I O N  T h e  s i g n a l  t o  b a c k g r o u n d  r a t i o  is s m a l l  f o r  C o m p t o n  t e l e s c o p e s  i n  a s t r o p h y s i -  c a l  e x p e r i m e n t s .  T h e r e f o r e  a g o o d  u n d e r s t a n d i n g  o f  t h e  b a c k g r o u n d  is e s s e n -  t i a l .  T h e  g o a l  o f  t h i s  s t u d y  is t o  p r o v i d e  a g o o d  m e a s u r e  f o r  t h e  a c t i v a t i o n  o f  t h e  C O M P T E L  i n s t r u m e n t ,  w h i c h  is h e l p f u l  f o r  t h e  c o n s t r u c t i o n  o f  b a c k -  g r o u n d  m o d e l s .  I n s t r u m e n t :  A d e t a i l e d  d e s c r i p t i o n  o f  C O M P T E L  o n b o a r d  C G R O  c a n  b e  f o u n d  i n  S c h S n f e l d e r  e t  al. (1993). C O M P T E L  c o n s i s t s  o f  t w o  s e t s  o f  d e t e c t o r  m o d u l e s .  A n  i n c o m i n g  g a m m a - r a y  p h o t o n  is C o m p t o n  s c a t t e r e d  i n  t h e  u p p e r  D1 d e t e c -  t o r  ( l i q u i d  s c i n t i l l a t o r )  a n d  a s s u m e d  t o  b e  t o t a l l y  a b s o r b e d  i n  t h e  l o w e r  D 2  d e t e c t o r  ( N a I ) .  T w o  c a l i b r a t i o n  u n i t s  ( " C a l - u n i t s " )  a r e  l o c a t e d  b e t w e e n  t h e  t w o  d e t e c t i o n  l a y e r s  a t  t h e  s i d e  o f  t h e  i n s t r u m e n t  o p p o s i t e  e a c h  o t h e r  ( S n e l l i n g  CP410, Proceedings o f  the Fourth Compton Symposium e d i t e d  b y  C .  D .  Definer, M .  S .  Strickman, and ] .  D .  K u f f e s s  9 1997 T h e  American Institute o f  Physics 1-56396-659-X/97/$10.00 1 5 7 7   This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to  IP:  132.177.229.79 On: Mon, 24 Nov 2014 16:49:54e t  a l . ,  1 9 8 6 ) .  T h e  C a l - u n i t  is c o m p r i s e d  o f  a 6 ~  d o p e d  p l a s t i c  s c i n t i l l a t i o n  w a f e r ,  v i e w e d  b y  t w o  1 / 2  i n c h  P M T s .  T h e  p r i m a r y  u s e  o f  t h e  C a l - u n i t  i s  t o  t a g  t h e  g a m m a - r a y s  f r o m  t h e  d e c a y  o f  6 ~  I n p u t  d a t a :  T h e  c o u n t  r a t e  o f  t h e  p l a s t i c  s c i n t i l l a t o r  i n s i d e  C a l - u n i t  B v a r i e s  c o n s i d e r a b l y  w i t h  r e s p e c t  t o  s e v e r a l  p a r a m e t e r s .  F i g u r e  1 s h o w s  t h e  v a r i a t i o n  o f  t h e  c o u n t  r a t e  o f  C a l - u n i t  B w i t h  t i m e .  T h e  e f f e c t  o f  t h e  s a t e l l i t e  r e b o o s t  a r o u n d  T J D  9 3 0 0  f r o m  a b o u t  3 3 0  k m  a b o v e  e a r t h  t o  a b o u t  4 4 0  k m  is c l e a r l y  v i s i b l e .  T h e  d e p e n d a n c e  o n  s o l a r  c y c l e  is s h o w n  i n  F i g u r e  2. T h e  b l a c k  d o t s  r e p r e s e n t  m e a s u r e m e n t s  a t  t h e  b e g i n n i n g  o f  t h e  m i s s i o n  a r o u n d  s o l a r  m a x i m u m ;  t h e  g r a y  d o t s ,  n e a r  s o l a r  m i n i m u m  i n  t h e  s a m e  a l t i t u d e  r a n g e .  N e u r a l  N e t w o r k  S i m u l a t i o n :  A N e u r a l  N e t w o r k  S i m u l a t i o n  w a s  c h o s e n  t o  m o d e l  tl~e c a l - u n i t  r a t e s  a s  a f u n c -  t i o n  o f  t h e  o r b i t a l  p a r a m e t e r s ,  b e c a u s e  m o s t  o f  t h e  f u n c t i o n a l  d e p e n d e n c i e s  a r e  u n k n o w n  a n d  b e c a u s e  t h e  p a r a m e t e r  s p a c e  i s  u n e v e n l y  f i l l e d .  M e a s u r e -  m e n t  n o i s e  a n d  d a t a  g a p s  p r e v e n t  u s e  o f  t h e  r a w  d a t a  i t s e l f  a s  a m o d e l  f o r  t h e  a c t i v a t i o n  o f  t h e  i n s t r u m e n t .  A N e u r a l  N e t w o r k  S i m u l a t i o n  M o d e l  w i t h  ? i n p u t  n e u r o n s ,  a n d  2 h i d d e n  l a y -  e r s  h 10 n e u r o n s  a n d  o n e  o u t p u t  n e u r o n  w a s  b u i l t .  T h e  a c t i v a t i o n  o ( j , n )  o f  t h e  n e u r o n  j i n  l a y e r  n e q u a l s  t h e  s i g m o i d a l  f u n c t i o n  F o f  t h e  w e i g h t e d  s u m  o f  t h e  F I G U R E  1. Measured count rates o f  Cal-unit B as a fimction o f  time. 1 5 7 8   This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to  IP:  132.177.229.79 On: Mon, 24 Nov 2014 16:49:54a c t i v a t i o n  f r o m  a l l  N n e u r o n s  i n  l a y e r  n-1 p l u s  a b i a s  b j :  ~v(n-,) i O(3,n ) --~ F (  E Wt7 " O( . . . .  1 )  + b ), w i t h  F ( x )  - 1 + e - ~  z = l  E a c h  i n p u t  n e u r o n  p r e s e n t s  o n e  o f  t h e  seven i n p u t  p a r a m e t e r s  as i t s  a c t i v a t i o n  t o  t h e  n e t w o r k .  T h e  i n p u t  values a r e  p r o p a g a t e d  t h r o u g h  t h e  n e t w o r k  u s i n g  t h e  f o r m u l a s  above. T h e  o u t p u t  o f  t h e  l a s t  n e u r o n  r e p r e s e n t s  t h e  m o d e l  v a l u e  c o r r e s p o n d i n g  t o  t h e  i n p u t  values. T h e  following m o d e l  p a r a m e t e r s  were cho- sen: h e i g h t  a b o v e  e a r t h ,  g e o g r a p h i c  l o n g i t u d e  a n d  l a t i t u d e ,  t i m e  since l a u n c h  ( t o  i n c l u d e  s o l a r  c y c l e  v a r i a t i o n s ) ,  a n d  t h e  o r i e n t a t i o n  o f  t h e  s a t e l l i t e  r e l a t i v e  t o  t h e  p r o p a g a t i o n  d i r e c t i o n  ( a z i m u t h ,  d e c l i n a t i o n ) ,  t o  a c c o u n t  f o r  a s y m m e -  t r i e s  i n  t h e  i n c o m i n g  p a r t i c l e  flux. To t a k e  c a r e  o f  t h e  d i s c o n t i n u i t y  o f  t h e  a z i m u t h  a n g l e  a t  360o/0 ~ , t h e  sine a n d  t h e  cosine o f  t h i s  a n g l e  a r e  u s e d  as i n p u t  p a r a m e t e r s .  D u r i n g  t h e  t r a i n i n g  o f  t h e  n e t w o r k  u s i n g  t h e  b a e k p r o p a g a -  t i o n  a l g o r i t h m ,  t h e  w e i g h t s  w i j  a n d  t h e  biases b j  a r e  o p t i m i z e d ,  m i n i m i z i n g  t h e  d e v i a t i o n  b e t w e e n  m o d e l  a n d  d a t a .  A s i m i l a r  a p p r o a c h  was m a d e  f o r  t h e  N e u r a l  N e t w o r k  s i m u l a t i o n  o f  t h e  b a c k g r o u n d  c o u n t  r a t e  o f  C O M P T E L  ( V a r e n d o r f f  e t  a l . ,  1996). M e a s u r e m e n t  o f  t h e  c a s c a d e  l i n e s :  A t  t h e  d e c a y  o f  22ha a 1.275 MeV p h o t o n  a n d / 3 + - p a r t i c l e  a r e  e m i t t e d  s i m u l -  t a n e o u s l y .  T h e  a n n i h i l a t i o n  o f  t h e  p o s i t r o n  ( a f t e r  i t s  d e c e l e r a t i o n )  p r o d u c e s  F I G U R E  2. Measured count rates of Cal-unit B during solar maximum (black dots) and solar minimum (gray dots). 1579  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to  IP:  132.177.229.79 On: Mon, 24 Nov 2014 16:49:54t w o  511 k e V  p h o t o n s  m o v i n g  i n  o p p o s i t e  d i r e c t i o n s .  O n e  o f  t h e  511 k e V  p h o -  t o n s  m a y  h i t  t h e  u p p e r  d e t e c t o r  (D1), w h i l e  t h e  1 . 2 7 5  M e V  p h o t o n  ( f r o m  t h e  s a m e  o r i g i n a l  d e c a y )  h i t s  t h e  lower d e t e c t o r  (D2). T h e  c o m p l e m e n t a r y  process ( 1 . 2 7 5  M e V  p h o t o n  i n  D 1 , 5 1 1  keV p h o t o n  i n  D2) is n o t  d e t e c t e d  d u e  t o  t h e  t h r e s h o l d  o f  t h e  D2 d e t e c t o r  ( 6 5 0  keV). T h e  m e a s u r e d  a p p a r e n t  t i m e  o f  f l i g h t  o f  s u c h  a n  e v e n t  is o n l y  s l i g h t l y  s m a l l e r  t h a n  t h e  t i m e  o f  f l i g h t  o f  a p h o t o n  w h i c h  is C o m p t o n  s c a t t e r e d  i n  D1 a n d  a b s o r b e d  i n  D2. A f t e r  t h e  d e c a y  o f  24Na a p h o t o n  a t  1.369 M e V  a n d  a p h o t o n  a t  2 . 7 5 4  M e V  a r e  e m i t t e d  s i m u l t a n e o u s l y .  A d e t a i l e d  d e s c r i p t i o n  o f  t h e  f i t t i n g  o f  t h e  c a s c a d e  l i n e s  (22Na a n d  24Na) u s i n g  s p e c t r a  o f  t h e  D2 d e t e c t o r  c a n  b e  f o u n d  i n  O b e r l a c k  e t  a l . ,  1997. R E S U L T S  M o d e l  d a t a  comparison: T h e  n u m b e r  o f  n u c l e i  g e n e r a t e d  as a f u n c t i o n  o f  t i m e  is t h e n  a s s u m e d  t o  b e  p r o p o r t i o n a l  t o  t h e  v a l u e  p r o d u c e d  b y  t h e  a c t i v a t i o n  m o d e l .  T h i s  is t h e n  f o l d e d  t o g e t h e r  w i t h  t h e  c o r r e s p o n d i n g  h a l f  life t i m e s  o f  t h e  isotopes. A c o m p a r i s o n  o f  t h e  m e a s u r e m e n t  o f  t h e  a c t i v a t i o n  o f  t h e  C O M P T E L  i n s t r u m e n t  a n d  t h e  m o d e l  is s h o w n  i n  F i g u r e  3 ( e a c h  d o t  r e p r e s e n t s  o n e  o b s e r v a t i o n  p e r i o d ) .  T h e  s c a l i n g  f a c t o r  f r o m  t h e  m e a s u r e d  C a l - u n i t  c o u n t  r a t e s  t o  t h e  n u m b e r  o f  22Na- a n d  ~4Na-nuclei as well as t h e  i n i t i a l  n u m b e r  o f  t h o s e  nuclei a r e  t a k e n  f r o m  a f i t  o f  t h e  m o d e l  t o  t h e  m e a s u r e d  d a t a .  T h e  s c a t t e r  i n  22Na is c a u s e d  b y  t h e  s t a t i s t i c a l  a n d  s y s t e m a t i c  e r r o r  o f  t h e  f i t  t o  t h e  d a t a .  T h e  a c t i v i t i o n  i n  24Na w i t h  a h a l f  life t i m e  o f  o n l y  15 h o u r s  shows m u c h  m o r e  s c a t t e r  t h a n  t h e  a c t i v a t i o n  i n  22Na. T h i s  is c a u s e d  b y  t h e  d i f f e r e n t  o r i e n t a t i o n s  o f  t h e  s a t e l l i t e  r e l a t i v e  t o  t h e  a n i s o t r o p i c  i n c i d e n t  p a r t i c l e  flux. T h e  s t r o n g  i n c r e a s e  o f  t h e  c a s c a d e  l i n e s  a f t e r  r e b o o s t  a r o u n d  T J D  9 3 0 0  is c l e a r l y  visible i n  b o t h  p l o t s .  F I G U R E  3. Comparison of the measured ~4Na- (left) and 22Na-activity (right) with the predicted values from the Neural Network Model. The difference between the two plots is determined by the drastic difference in isotope lifetimes (15 h for ~4Na, 2.6 years for 22Na). 1580  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to  IP:  132.177.229.79 On: Mon, 24 Nov 2014 16:49:54T h e  r e c e n t  r e b o o s t  t o  a h i e g h t  o f  a b o u t  517 k m  will i n c r e a s e  t h e  c o m b i n e d  b a c k g r o u n d  f r o m  2 2 N a  a n d  2 4 N a  b y  a f a c t o r  o f  2 t o  3 i n  t h e  n e x t  1 t o  2 y e a r s .  E s t i m a t i o n  o f  t h e  a v e r a g e  i n c i d e n t  p r o t o n  f l u x :  T h e  r a d i o a c t i v e  i s o t o p e  2~Na is p r o d u c e d  b y  t h e  i n t e r a c t i o n  o f  e n e r g e t i c  p r o -  t o n s  w i t h  A l - a t o m s  i n  t h e  s t r u c t u r e  o f  t h e  s a t e l l i t e .  T h e  m e a s u r e d  t i m e  o f  f l i g h t  d i s t r i b u t i o n  o f  t h e  e v e n t s  f r o m  t h e  2 2 N a  d e c a y  s h o w s ,  t h a t  m a i n l y  e v e n t s  f r o m  t h e  s t r u c t u r e  n e a r  t h e  u p p e r  d e t e c t o r  ( D 1 )  c o n t r i b u t e  t o  t h e  signal. T h e  A l - m a s s  a r o u n d  D1 is a p p r o x i m a t e l y  125 k g  w i t h  a t h i c k n e s s  i n  t h e  r a n g e  o f  1 c m  t o  2 c m .  T h e  a v e r a g e  d a i l y  p r o t o n  f l u e n c e  is t h e n  e s t i m a t e d  b y  c o m p a r i n g  t h e  d a i l y  i n c r e a s e  i n  t h e  n u m b e r  o f  p r o t o n  p r o d u c e d  n u c l e i  w i t h  a n  e s t i m a t e  o f  t h e  e f f i c i e n c y  o f  t h e  p r o d u c t i o n  o f  t h e s e  n u c l e i  b y  p r o t o n s .  T o  s i m p l i f y  t h e  c a l c u l a t i o n ,  a l l  t h e  A1 m a s s  w a s  p u t  i n  a p l a t e  o f  2 c m  t h i c k n e s s  a n d  i r r a -  d i a t e d  b y  p r o t o n s  f o l l o w i n g  t h e  s p e c t r a l  s h a p e  o f  a m o d e l  c a l c u l a t i o n  o f  t h e  o r b i t a l  p r o t o n  s p e c t r u m  ( S t a s s i n o p o u l o s ,  1981). A t  t h e  b e g i n n i n g  o f  t h e  m i s -  s i o n  d u r i n g  s o l a r  m a x i m u m  a t  a n  o r b i t a l  a l t i t u d e  o f  440 k m  a d a i l y  p r o d u c t i o n  o f  1.7 9 10 ~ n u c l e i  o f  2 2 N a  w a s  d e r i v e d  f r o m  m e a s u r e m e n t s  o f  t h e  2 2 N a  c o u n t  r a t e .  A d a i l y  p r o t o n  f l u e n c e  ( E > 100 M e V )  o f  2.3 9 10 s p r o t o n s / c m  2 c a n  b e  i n f e r r e d  f r o m  t h o s e  n u m b e r s .  F o r  a n  a l t i t u d e  o f  462 k m  a d a i l y  f l u e n c e  o f  5 . 1 0 5  p r o t o n s / c m  2 w a s  p r e d i c t e d  b y  D y e r  e t  al. (1994). A c k n o w l e d g e m e n t s  W e  w o u l d  l i k e  t o  t h a n k  t h e  M I T R E  C o r p o r a t i o n  f o r  p r o v i d i n g  t h e  A s -  p i r i n / M I G R A I N E S  s o f t w a r e  p a c k a g e .  T h e  C O M P T E L  p r o j e c t  is s u p p o r t e d  b y  t h e  G e r m a n  g o v e r n m e n t  t h r o u g h  D A R A  g r a n t  50 Q V  90968, b y  N A S A  u n -  d e r  c o n t r a c t  N A S S - 2 6 6 4 5  a n d  b y  t h e  N e t h e r l a n d s  O r g a n i s a t i o n  f o r  S c i e n t i f i c  R e s e a r c h  ( N W O ) .  R E F E R E N C E S  1. SchSnfelder, V., A a r t s ,  H., B e n n e t t ,  K., e t  al., 1 9 9 3 ,  Astrophysical J o u r n a l  Suppl. 8 6 , 6 5 7 .  2. Shelling, M., e t  al., 1986, Nuclear I n s t r u m e n t s  & Methods, A 2 4 8 ,  545-549 3. Oberlack, U., e t  al., "A New Background Model for C O M P T E L  1.8 MeV D a t a " ,  t h e s e  Proceedings, P o s t e r  20-5. 4. Varendorff, M., Forrest, D., McConnell, M., and R y a n ,  J . ,  1 9 9 6 ,  A&A Suppl., 1 2 0 ,  699-702. 5. Stassinopoulos, E . G . ,  " G R O  charged particle radiation s t u d y " ,  N A S A  R e p o r t  X-601-81-38, Table 3. 6. D y e r ,  C. S., T r u s c o t t ,  P. R . ,  Evans, H. E., Hammond, N., Comber, C., B a t t e r s b y ,  S., 1994, I E E E  Transactions on Nuclear Science, V o l .  3, 438. 1581  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to  IP:  132.177.229.79 On: Mon, 24 Nov 2014 16:49:54

A time dependent model for the activation of COMPTEL

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A time dependent model for the activation of COMPTEL

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