PDS_VERSION_ID    = PDS3
RECORD_TYPE       = STREAM
OBJECT = TEXT
  NOTE = "HDAC Calibration Notes."
  PUBLICATION_DATE = 2004-09-20
END_OBJECT = TEXT
END
Procedure for HDAC Calibration
Ralf Reulke
July 30, 2005

Background: 

The Cassini HDAC consists of a Channel Electron Multiplier (CEM) photon 
detector equipped with 3 absorption cell filters: a hydrogen cell, a 
deuterium cell, and an oxygen cell. The cells are separated by MgF2 windows. 
(The oxygen cell used to suppress the red part of the spectrum was vented 
prior to flight.) The hydrogen and deuterium cells function as adjustable 
absorption filters. In each cell, a hot tungsten filament dissociates  the 
H2 and D2 molecules into atoms, producing an atomic density dependent on the 
filament temperature. These atoms resonantly absorb the hydrogen and 
deuterium Lyman-alpha lines (located at 1215.67  and 1215.34 ) passing 
through the cells.


The optical depth of an HDAC cell at line center depends on the photo 
absorption cross section, the number density of the atoms, and the tube 
length. The total signal is expressed as 

I = INTEGRAL( S(l) H(l) T(l) dl )

where I is the signal seen by the photon detector, H(l) is the intensity of 
the radiation as function of wavelength l, S(l) is the spectral sensitivity 
of the photon detector, and T(l) is the spectral transmission of the filter 
(l is the wavelength). The ratio of the modulated intensity I to the 
unmodulated intensity I0 (assuming no background) is given by the reduction 
factor R = I / I0. I0 is the background level, which can be determined by 
taking the differences between the modulated and unmodulated signals.

The spectral sensitivity of the photometer is described in the following 
table.

		wavelength   sensitivitiy 
		[Angstroem]  (normalized)

       0       1115.0000    0.0011701495
       1       1170.0000      0.76815438
       2       1210.0000      0.91592476
       3       1250.0000       1.0000000
       4       1290.0000      0.95614865
       5       1330.0000      0.88525059
       6       1370.0000      0.70129334
       7       1410.0000      0.55144270
       8       1450.0000      0.32617267
       9       1490.0000      0.18712511
      10       1500.0000      0.16523713
      11       1600.0000     0.064720643
      12       1700.0000     0.021050599
      13       1800.0000    0.0056041709


When no power is applied to the filament, the absorption cells contain 
only molecules, and the cells are transparent to incoming radiation at 
Lyman-alpha.

Three different measurement modes are possible:

1. Measurement in the photometer mode.

HDAC in photometer mode detects the radiation in dependence of its spectral 
sensitivity.  For pure H-Lyman Alpha radiation (e.g. LISM) the sensitivity 
of HDAC is about 50 count/s/Rayleigh.

2. Spectral retrival of H-Lyman Alpha line

D- and H-cell modulations of the H-Lyman Alpha radiation yield the 
characteristic parameters of the H-line like strength and width. Following 
parameter set should be used for the power-tau relation:

tauHH_fit=[0,0.0024,0.0468,0.0850,0.1073,0.4081,0.6554,0.8655]
tauHD_fit=[0,0,0,0,0,0,0,0]
tauDD_fit=[0,0.3,1.5,2.8,4.0,6.2,7.5,10.0]
tauDH_fit=[0,0.02,0.16,0.25,0.37,0.87,1.06,1.28]

The table shows, that the modulation of D-cell has some influence on 
H-Lyman Alpha modulation.

3. D/H retrival

Cycling the filaments on and off and comparing the differences in signal 
gives a direct measurement of the relative hydrogen and deuterium signals.