HEAO-1 Software:
The Discovery Scalars of the A2 Detectors


Based on information from "HEAO-1 A-2 Experiment: The Configuration of the Hard X-ray Detectors" F.E. Marshall 1983.

The discovery scalars are the eight rates (four from large field of view data and four from small field of view data) accumulated in various combinations of spatial and spectral windows.

The HEDs were a composite of two layers (M1 and M2) consisting of a single row of anodes each. Alternate groups of anodes were under different collimators. Anodes under the 3.0° x 3.0° collimators were known as L1 (for layer one) and L2 (for layer two). Anodes under the other collimator were called R1 and R2. The final layer of anodes generated the veto signal V1. For HED-1 and HED-3, the side cells were also included in the V1 signal. For HED-2 and the MED which lacked the front veto layer, V1 consisted of the bottom layer and V2 consisted of the two side rows.

The first four discovery scalars were fixed and unchanging throughout the course of the mission and were defined to be L1, R1, L2, and R2 in order. These discovery scalars identified also the small and large FOV as follows:

HED-1 & HED-2 HED-3 & MED
Large FOVR1 & R2L1 & L2
Small FOVL1 & L2R1 & R2
For example the Large FOV for the HED-3 detector would correspond to selecting discovery scalars 1 and 3. The small FOV would correspond to 2 and 4.

Discovery scalars 5 and 7 are for the left; scalars 6 and 8 are for the right. Table 1 below gives the approximate energy ranges for the six windows for the HEDs (which were nominally identical) and the MED.


Table 1: Spectral Windows for HEAO-1 A2 Discovery Scalars

Window MED energy (keV) HED energy (keV)
1A 1.5 - 6 2 - 6
1B 6 - 8 6 - 8
1C 8 - 10 8 - 32
1D 10 - 20 32 - 60
2A 2 - 3.9 9 - 32
2B 3.9 - 20 32 - 60

The definitions for discovery scalars 5-8 changed throughout the course of the missions and were formed from combinations of six pre-designed spectral windows: Layer 1 - 1A, 1B, 1C, and 1D; Layer 2 - 2A and 2B. Table 2 lists the different configurations of discovery scalars 5 through 8 over the course of the mission.


Table 2: History of Discovery Scalar Definitions

MED HED-1 HED-2 HED-3
Day
of 1977
DS5,6 DS7,8 DS5,6 DS7,8 DS5,6 DS7,8 DS5,6 DS7,8
232.04 1A Veto 1A Veto 1A Veto 1A Veto
234.94 " " 1B 1D+2B 1B 1D+2B 1B 1D+2B
238.89 1B 2B " " " " " "
242.20 " 1D " " " " " "
248.15 " 2B " " " " " "
304.93 " " " 1C " 1C " 1C
321.79 " " " 1C+1D " 1C+1D " 1C+1D
476.20 " " " " " " 1A "
488.82 " " " " " " 1B "
615.27 " 1C+1D " " " " " "


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