112 hits in 5 planes is 22 hits per plane not very different from 39 hits per plane in the TPG. the big difference is that there are 
100 signa hits in tpg vs 5 hits in the scifi. 

an approximate figure of merit of S/sqrt(B) would give 

100/sqrt(4000)= 1.3 vs 5/sqrt(100) =0.5 

one way is 2 times worse the other is twice better... 




3. Sensitivity to background

The long integration time of the TPC is compensated by the small amount
of material and by the large number of samplings on each track. The
total amount of sensitive material in the TPG is 0.01 g/cm2 to be
compared with a total of 0.8 g/cm2 for the scintillating fiber tracker,
with material having a similar absorption length for photons. (The
spectrum will be cut-off at about 10 keV by the absorption in the
aluminum windows). Thus the background rate would be smaller in the TPG by a factor of the order of 80.The integration time of the TPG is 50 microseconds against 5 nanoseconds(?) for the fiber tracker, so the backround during sensitive time is 125 times worse in the TPG. However there are 100 points in the TPG against 5 in the Sci-Fi so the ratio of bkg hits per signal hit is 120/20=6 times worse in the TPG than in the Sci-Fi. (this becomes 1.5 if integration time is 20 ns in sci-fi)


The TPG will give more than 100 points so that the effect of the larger background per point should be compensated by the higher redundancy of the detector. 

A better figure of merit may be S/sqrt(B) which is in this case in favor of the TPG by a factor 
100/5 / (sqrt(0.01/0.8)/sqrt(510^4/5)
20 / sqrt(1/80)/sqrt(10^4) 
20*9/100 =1.8
this time in favor of the TPG. 

absolute rates are more questionables, but let me give a try. 
mass of TPG gas is 0.1 g/l*70700 cm3 (70 liters) = 7 grams
reference of 1 MHz in 0.1g gives 70 MHz in 7 grams, 
i.e. in 50 microseconds: 70*50=3500 hits this is very close from your estimate. 
the total mass of the Scifi is 0.8*707 or 560 grams. 
the total rate in scifi for that same flux is then 5600 MHz, or 5.6 per nanosecond. This gives indeed 112 in 20 ns, (or  28 in 5 ns).  
  
     
incidently the higher mass of the Sci-fi will give more multiple scattering. 

Hi,

Well, one more try.  

Fundamental parameters:

Polystyrene:    rho=1g/cm^3
                        mu/rho = 2 cm^2/g  @10keV
                                   0.20      @50keV
                                   0.16  @100keV  

He (gas @STP)   rho=.09g/L
                        mu/rho = 0.2 cm^2/g @10keV
                                   0.17   @50keV
                                   0.15       @100keV

Take Lab G measurement:

0.835 mm diameter fiber, 30 cm long

mass = 0.164g
Area = 2.0 cm^2  (area presented to x-ray beam, approx)

This gives: 0.082g/cm^2

Measured rate was say, 1 MHz

Lets use the 50keV attenuation coefficients.

Note:  Lab G measurement did not have an absorber.
However, 40 cm of LH only reduces the flux by about 70% 
at 10keV. For H (10keV) we have Atten = exp[-2.8g/cm^2 X .4cm^2/g] =
0.33 So the effect is not enormous plus for the fibers a 10keV x-ray 
absorption is probably below our threshold.

So back to the calc - 

The rate in the fiber is taken as 1MHz, then

1MHz = No[1-exp(-.2cm^2/g X 0.082g/cm^2)] = No(0.016)

No=61 MHz

Then for the TPG (assuming He at STP)

1 cm^2 cross section by 100 cm drift length is 0.009g/cm^2 (.1L)

Then

N(hits in the chamber per 60 microsec drift time) =

61E6 X 60E-6 X [1-exp(-.17cm^/g X .009g/cm^2)] X 707cm^2(chamber
area)=3900.

[I had the g/cm^2 of a He cell too high by a factor of 10 in my last
calc.]

So there are roughly 40 hits per "measurement plane in the TPG"

It would be interesting for Giles to run his analysis on this.

Some comments:

1. The x-ray energy spectrum could have a large impact, particularly
   for the TPG.  The fibers are less sensitive since at around 10 keV
   we don't register a hit.

2. These are only numbers for x-ray conversions.  The real situation
will be
   worse since each x-ray conversion can deposit charge in more than one
cell
   in the device.  This might be particularly bad for the TPG, although
it 
   might be a non-negligible effect in the fibers also. 


Please check my assumptions and numbers.

Alan




I should have given the equivalent for the fiber tracker.

Assuming 350 micron doublets, 3 views/station, and 5 stations per spect,

I get for the FT:

N = 61E6 X 20E-9 X [1-exp(-.2cm^2/g X .7g/cm^2)] X 707 = 112

Alan