What are Overshoots?

Overview

This thread introduces the concept of NICER's overshoot events. We discuss what overshoots are, and how they arise. In short, overshoots are typically caused by charged particles that deposit a large amount of charge. Thus, the overshoot count rate represents NICER's background components due to particle background.

Read this thread if you want to: Understand more about overshoots

Last update: 2021-07-20

Introduction

NICER detectors are known as Silicon Drift Detectors, or SDDs. SDDs are fast-response silicon detectors that enable NICER's high throughput and fast temporal response, while maintaining CCD-like spectral resolution.

NICER's detectors are in a space environment where charged particles are plentiful. Charged particles may include cosmic rays, i.e. high energy charged particles from outside the geomagnetosphere, but also may include trapped charges (radiation belts) such as SAA and what NICER typically calls the "polar horns."

Most high energy particles pass through NICER's SDD active silicon volume, but deposit charge along its path. This leads to a large amount of injected charge, larger than a typical X-ray, which is registered as an overshoot event.

Please note that overshoots are quite different from undershoots. Undershoots are discussed on the What Are Undershoots And Optical Loading? analysis thread page.

How SDD Detectors Work

NICER's Silicon Drift Detectors (SDDs) are circular silicon-technology devices that are sensitive to X-ray photons. X-ray photons enter the silicon device and convert to electrons where they drift to the read-out anode and are recorded. Figure 1 shows a highly simplified diagram of an SDD.

Figure 1.Simplified cross section diagram of a NICER SDD showing the silicon (blue) with various electrode structures (pink). Electrons drift along field lines (dashed blue) to the anode where they are amplified. Electron hole pairs (dot-circle pairs) can be created by X-ray photons and charged particles.

A key aspect is that while both X-rays and charged particles inject electron-hole pairs into the active silicon volume, charged particles most often inject an order of magnitude more than X-rays. Electrons and protons in NICER's space environment that pass straight through the entire thickness of the detector inject 200-300 keV (or more) worth of charge into the volume. Compared to the NICER upper level discriminator threshold of ~20 keV, and the maximum pass-band of the X-ray concentrators of ~13 keV, charged particle energy deposition is far above a standard X-ray. Hence, they are designated as "overshoots."

As a side note, charged particles moving diagonally through the silicon volume may inject more than the minimum charge, since the path length is larger than the thickness. But charged particles just barely clipping the detector corners may inject a small enough amount of charge that it can be detected as a "normal" event. It is these events that are the source of non X-ray backgrounds within NICER.

Where Are Overshoots Recorded?

Overshoot reset events are stored in the event list just as normal X-rays are stored. They are located in the raw per-MPU event files, also known as the "uf" (unfiltered) event lists, in the xti/event_uf/ subdirectory of each observation. Each event has an overshoot flag bit set in the FLAGS column of the event list.

Overshoots are an interesting enough diagnostic that their count rate is also stored in the per-observation filter file. See Managing NICER Filter Files for more information about NICER filter files. A key quantity in the filter file is FPM_OVERONLY_COUNT, which is the average per-detector overshoot rate.

Why Do We Care About Overshoots?

Overshoots are due to charged particles, which are the same charged particles that create non-X-ray backgrounds within the NICER detectors. Thus, tracking the overshoot rates is important for background reduction purposes.

The standard NICER filtering sets upper limits to the overshoot rate, thus limiting the maximum contribution of non-X-ray backgrounds.

What Are The Sources of OverShoots?

As discussed above there are several souces of overshoots.

• Cosmic Rays. These are charged particles accelerated outside the geomagnetosphere, typically outside the solar system, that penetrate to the ISS orbit. True cosmic rays are modulated by the Cutoff Rigidity (COR) value, and overshoots due to this contribution vary in the 0.1-1 overshoot ct/s.
• Trapped Charges (SAA). The inner radiation belt is primarily composed of protons, and is also known as the South Atlantic Anomaly (SAA). This belt is usually stable, although extreme solar events may alter it. Overshoots due to SAA events vary while passing through the SAA region, and max contribute up to 100 overshoot ct/s at maximum.
• Trapped Charges (Polar Horns). The outer radiation belt is primarily composed of electrons, and is known in the NICER community as "Polar Horns." These belts will be found at the highest latitudes. They are also highly variable. Often, the polar horns are a negligible source of overshoots, but in extreme cases may contribute thousands of overshoot ct/s per detector.
• Solar Energetic Particles. Extreme solar events may produce energetic particles that penetrate to the ISS orbit.
• X-rays with energy >20 keV. True X-rays above the 20 keV limit are rare since NICER's X-ray Concentrators (XRCs) have negligible reflectivity for such X-rays.

What Comes Next?

Please see the NICER Overshoot Filtering Recommendations analysis thread for more discussion on screening strategies for overshoots.

Modifications

• 2021-07-20 - initial draft