The HENA sensor can be divided into a micro-channel plate (MCP) half and a solid-state detector (SSD) half. The overall HENA sensor consists of start, stop, coincidence, and solid-state detectors. The start, stop, and coincidence detectors make up the MCP sensor (HENA-M). The start, coincidence, and solid-state detector make up the solid-state sensor (HENA-S). A particle's trajectory determines whether it hits the stop detector or the solid-state detector, and thus whether it is processed by HENA-M or HENA-S. The two HENA sensors are complementary. HENA-S produces a more reliable energy determination than HENA-M, but HENA-M has higher spatial resolution. The two sensors also provide some fault-tolerance via the redundancy.

The HENA-M sensor measures the trajectory and velocity of energetic particles. During nominal operations, alternate collimator plates are charged to high positive and negative voltages; the collimators sweep up ions and electrons entering the sensor. Neutral atoms pass by the collimators through a slit. First, the particle hits a foil, dislodging secondary electrons. The electrons are accelerated towards the start detector. The start detector has one dimensional imaging and consists of an MCP and anode. Meanwhile, the particle continues on towards the stop detector. The stop detector images in two dimensions and consists of an MCP and anode. In front of the stop detector is another foil. The particle dislodges electrons from this foil too. Some electrons are accelerated towards the stop MCP. Backscattered electrons are accelerated to the coincidence MCP. The start and stop detectors measure particle position and velocity. The coincidence detector helps confirm particle validity.

The HENA-S sensor measures the trajectory, energy, and velocity of energetic particles. The sensor shares the collimators and the start detector with HENA-M. When the particle hits the solid-state detector, electrons are backscattered and picked up by the coincidence MCP. The solid-state detector measures particle energy, and with the start detector, measures position. The coincidence detector acts as a stop detector in HENA-S, and with the start detector, measures velocity.

In front of the slit is the shutter, which is normally open. The shutter can be commanded closed. When closed, the slit is replaced by a small square opening and two radioactive calibration sources are placed over the entrance slit.

The HENA boresight vector is defined as the vector normal to the stop detector passing through the center of the slit. The HENA coordinate system is defined by the boresight vector: the vector is at zero degrees azimuth and has zero degrees elevation. The HENA boresight vector is located 135° from the spacecraft (S/C) +X axis rotated about the +Z (spin) axis. A two dimensional coordinate system is defined for talking about the HENA detectors; see the sensor figure above for the relationship between the spacecraft coordinate system, the HENA sensor, and the HENA coordinate system. All subsequent coordinate system usage refers to HENA coordinates unless spacecraft coordinates are explicitly used. The HENA-M and HENA-S sensors have contiguous 45° by 120° fields of view. The combined HENA sensor's field of view is 90° in the S/C X-Y plane and 120° along the S/C Z axis centered on the boresight vector (again, see figure).

The start detector measures where along the slit the particle entered the sensor. When electrons hit the start detector, the MCP multiplies the electrons a million-fold. The electrons spread out into a small area before falling on the anode. The anode consists of a pair of interdigitated wedges. If the electrons fall on the tip of a wedge, a small pulse is generated; if the electrons fall nearer the base of a wedge, a larger pulse is generated. The wedges are oriented such that the larger the pulse on the bottom wedge, the farther the particle was along the X axis (yes, the bottom is above the top). In other words, the X axis position is proportional to Wfb/(Wft + Wfb).

The start detector also roughly measures the energy of the particle. The more energetic the particle, the more electrons are knocked off the front foil and the more electrons reach the anode. The sum of the wedges' pulses are proportional to the energy of the particle. Because the number of electrons generated are described by Poisson statistics, and the numbers are small, this measure of energy is imprecise.

The stop detector measures where the particle hits in two dimensions. A two-dimensional MCP magnifies the electrons so that a cloud of electrons hits the anode. The anode consists of wedges and strips. The strips are successively larger across the anode. The larger the strip pulse (Sb), the farther the particle was along the X axis. The larger the wedge pulse (Wb), the farther the particle was along the Y axis. Electrons that hit neither wedge nor strip contribute to an interstitial pulse (Ib). The X axis position is proportional to Sb/(Wb + Sb + Ib) and the Y axis position is proportional to Wb/(Wb + Sb + Ib). The energy of the particle can also be determined from the sum of the pulses.

The SSD detector is mounted in 2 different planes (see HENA schematic, above). It consists of 4 physical wafers, each pixelated into 60 pixels and arranged such that there are 24 pixels in the dimension that defines elevation (X), and 10 pixels in the dimension which defines the azimuth (Y). The SSD detector produces an analog energy pulse and an 8 bit digital pixel ID for each event. These are routed to the HENA event logic boards, where the data are combined with inputs from the Start and Coincidence MCPs and checked for validity. The energy pulse is then digitized and passed along with the pixel ID to the DPU, where the pixel ID is converted to a backplane position for use in computing the particle trajectory, and the energy is combined with the TOF to compute the particle mass.

In addition to generating individual events, the SSD detector accumulates a 16-bin energy spectrum for each of the 240 pixels in 16 bit deep bins. This data can be treated as 16 images, each image representing a different energy. The images can be optionally read by the DPU. Reading the images clears them.

Return to HENA Software User's Guide. Report problems to John Hayes.