Revised 21 May, 1996



		THE MONTE APERTURE MATERIALS MODEL

			Known Components

1)  Thermal Blanket - Based upon information from Oren Scheinman,
    we assumed the following composition for the blanket:

	.0025 cm Teflon (silver coated over Inconel)
	.0051 cm Kapton ("metalized", will be modeled as mylar)
	.0102 cm Mylar (in 16 aluminized layers)
 
    We have since obtained a sample that replicates the flight
    blanket and measured its absorption.  The results of these
    measurements may be characterized by the following components:

	.015 cm Mylar
	.0025 cm Teflon
	.000015 cm Silver
	.000035 cm Aluminum
	.00001 cm Inconel (80% Ni, 14% Cr, 6% Fe)

These have the following properties:

Substance    Path      uabs @ 10  uscat @ 100  Atten@10keV
	   (gm/cm2)     (cm2/gm)    (cm2/gm)	   (%)

 Mylar	     0.021	 2.962       0.133	   6.03

 Teflon      0.0055	 6.188	     0.123	   3.34

 Silver	     0.00016	 116.0       0.130	   1.81

Aluminum     0.00009      25.7       0.143  	   0.24
 
 Inconel     0.00009	 198.1       0.141	   1.67

  
Total attenuation at 10 keV is 12.8%.  This extrapolates to 1.7% at
20 keV.  Total scattering probability at any energy is about 0.35%.
A postscript plot of Rick's analysis of the blanket (x2 thickness)
is available at /home/rr/ioc/idl.ps.


2)  Detector Components


    Berylium Window (.020" Be plus following impurities):
	620 ppm by weight of Fe
	350 ppm by weight of Ni

    Mylar Cushion ( .002" @ 1.395 gm/cm3) C10H8O4

    Teflon Reflector/Cushion (.008" @ 2.2 gm/cm3) CF2


These have the following properties:

Substance    Path      uabs @ 10  uscat @ 100  Atten@10keV
	   (gm/cm2)     (cm2/gm)    (cm2/gm)	   (%)

   Be	   0.09144	 0.401	     0.132	   3.60

   Fe	   0.00006	 169.0	     0.138	   1.01

   Ni	   0.00003	 211.0	     0.142	   0.63

 Mylar	   0.00709 	 2.962	     0.133	   2.08
 
Teflon     0.04470	 6.188	     0.123	   24.1


Total attenuation at 10 keV is 29.6%.  This extrapolates to 4.3% at
20 keV.  Total scattering probability at any energy is about 1.8%.


                      Speculative Components

We currently believe that only the only component in the aperture
that remains quantitatively unknown is the NaI dead layer which is
expected to vary from detector to detector.  The values for the
dead layer thicknesses, listed below, were estimated by Len using
Leonardo on the basis of the excess attenuation evident after
taking into account the known components (listed above).  It should
be noted; however, that the assessment of the known components used
by Len predates the latest information on the aperture blanket.
Thus we should regard these values for the dead layer thicknesses
as provisional and anticipate that they will be updated.  Also,
Len's assessment of the dead layer thicknesses properly made no
assumption about other possible mechanisms for altering the shape
of the observed count spectrum at low energies.  We have evidence
that, at least for the initial 300 us XULD window, there is a
deficit of counts for channels <30.  We are investigating this
effect separately and will update the dead layer estimates, if
appropriate.

The current dead layer estimates follow:

   Detector		Dead Layer Thickness    
			     (microns) 

      A0			6.02
      A1			8.38
      A2			11.6
      A3			5.81
      B0			13.5
      B1			9.14
      B2			6.71
      B3			4.23


		   THE MONTE MODEL

I have modeled the HEXTE aperture absorption by accurately
simulating the two high Z components (Silver and NaI) so that the
relevant absorption edges are represented and by approximating the
low Z components as a single layer of Carbon.  I list below the
MONTE model of the aperture components and summarize their
absorption and scattering properties relative to an ideal model
that would represent each known element exactly.  For simplicity, I
specify an average value for the dead layer thickness of 8.2
micron.  The actual monte model uses the dead layers specified
above for each individual detector.

Element	    Path      uabs @ 10  uscat @ 100  Atten@10keV
	  (gm/cm2)     (cm2/gm)    (cm2/gm)	   (%)

   NaI	  .00303	 139.4	    0.127	  34.5

   Ag	  .00016	 116.0	    0.130	  1.81	  

    C	  .24440      	  1.91      0.148     	  37.3

Total attenuation at 10 keV is 59.7%.  This extrapolates to 10.7%
at 20 keV.  Total scattering probability at any energy is about
3.6%.  These may be compared to the ideal values of 59.7%, 10.7%
and 2.2% for the 10 keV absorption, 20 keV absorption and
scattering probabilites respectively.  Thus, the main inaccuracy
in the model is that it gives ~1.4% too much scattering.  This
should not be significant since only about one-half of the scattered
component is removed from the beam.

The Iodine and Silver K edges give absorption discontinuities of
7.4% and 0.8% in the NaI dead layer (8.2u thickness) and the silver
blanket coating (.15u thickness) respectively.