Proportional counters and microchannel plates Sebastian Diebold Abstract Developed right at the beginning of the space age in the 1940s the pro-

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Proportional counters and microchannel plates

Sebastian Diebold ?

Abstract Developed right at the beginning of the space age in the 1940s, the pro-

portional counter was the ﬁrst detector used in X-ray astronomy and stayed its

workhorse for almost four decades. Although the principle of such a detector seems

to be rather simple, over time it underwent considerable performance improvements

and the lifetime under orbital conditions was extended tremendously. Particularly

the invention of position-sensitive proportional counters provided new and sophis-

ticated methods to discriminate background and thus enabled observations of much

weaker sources.

A leap forward in position resolution was achieved with the advent of microchannel

plate (MCP) detectors in the 1970s. In contrary to gas ﬁlled detectors, they provide

no considerable energy resolution but feature spatial resolutions reaching down to a

few tens of micrometers, ﬁtting ideally the angular resolution of the novel grazing

incidence imaging X-ray telescopes upcoming at that time.

Even today, both types of detectors are still relevant in space-based astronomy. How-

ever, in case of MCPs new developments focus on the far and extreme ultraviolet

wavelength range, while the Chandra X-ray observatory is most likely the last mis-

sion applying this technology for X-rays. In contrast, compact detectors with gas

electron multiplier (GEM) foils and micropattern readout are currently under heavy

development for the soft X-ray range, since they allow for the ﬁrst time to measure

polarization in X-rays over a broad energy range.

This chapter presents the principles of proportional counters and MCP detectors,

highlights the respective performance characteristics, and summarizes their most

important applications in X-ray astronomy.

Sebastian Diebold

Institut f¨

ur Astronomie und Astrophysik, Eberhard Karls Universit¨

at T¨

ubingen, Sand 1, 72076

T¨

ubingen, e-mail: diebold@astro.uni-tuebingen.de

?corresponding author

arXiv:2210.10883v2 [astro-ph.IM] 4 Dec 2022

2 Sebastian Diebold

Keywords

X-ray astronomy, instrumentation, radiation detectors, proportional counters, imag-

ing proportional counters, microchannel plate detectors

1Introduction

The history of gas-ﬁlled detectors dates back to the onset of nuclear physics at the

beginning of the twentieth century with the studies on gas ionization by Thom-

son (1899) and the ﬁrst detector implementations by Rutherford and Geiger (1908).

Shortly after, in 1912, such detectors led to the discovery of cosmic rays when Hess

(1912) used them on several balloon ﬂights to investigate the origin of natural radi-

ation.

The proportional counter was introduced in the late 1940s, followed by years of

intensive further development mainly driven by its applications in particle physics

(Knoll, 2010). This phase proceeded until the introduction of NaI scintillation de-

tectors in the 1950s and the ﬁrst semiconductor detectors in the 1960s. However, the

developments on proportional counters continued and lead to the position-sensitive

single wire proportional counter (SWPC) and to multi-wire detectors that feature

two dimensional imaging capabilities (Fraser, 2009).

From the ﬁrst rocket missions investigating the X-ray radiation from the sun in

1948 (Keller, 1995), gas-ﬁlled detectors and particularly proportional counters were

for almost four decades the workhorse in imaging as in non-imaging soft X-ray

astronomy (Pfeffermann, 2008b). In 1962, the ﬁrst celestial X-ray source outside

the solar system was discovered with a proportional counter on a sounding rocket

with a remarkable energy resolution of about 20 % at 6 keV. This success hold on

and for two decades all main advantages in the ﬁeld were due to missions applying

proportional counters (Bulgarelli and Guainazzi, 2020). A milestone in the ﬁeld

that has to be mentioned was the ﬁrst satellite mission fully dedicated to X-ray

astronomy: the Uhuru satellite launched in 1970 (Giacconi et al., 1971). It featured a

proportional counter sensitive in the band 2–20 keV with a relatively large sensitive

area of 0.084m2and an angular resolution of 1◦×10◦FWHM constrained by a

collimator.

According to Fraser (2009), in X-ray astronomy the main aspects of proportional

counters compared to other detectors can be summarized in these three points:

1. relatively large active effective area,

2. moderate energy and spatial resolution, and

3. high sensitivity.

Microchannel plates (MCP) – the second detector technology that is covered in

this chapter – started as a military development for night vision devices, but was

declassiﬁed and MCPs became commercially available. The main application for

the MCP technology is the position-resolved detection of charged particles with

Proportional counters and microchannel plates 3

a very low energy threshold, but it is also an excellent option for photon detec-

tion when an appropriate photocathode for the wavelength range of interest is used.

Since the 1980s, MCPs were for many years the main technology for astronomi-

cal instruments over the complete ultraviolet (UV) spectral range and they are still

competitive in this waveband to silicon-based detectors like CCDs or CMOS. In

fact, the best technology for a certain application depends strongly on the individual

instrumental requirements and the parameters of the satellite platform. Not only in

the UV, but also in the visible and the X-ray bands MCP detectors were success-

fully applied. However, new instruments and missions with MCP detectors are only

proposed in the far (FUV) and extreme UV (EUV, sometimes also XUV).

Considering X-ray astronomy, the key feature of MCP detectors is the unprece-

dented position resolution. Therefore, it is no wonder that with HRI (High Reso-

lution Imager) (Kellogg et al., 1976) ﬂown on the Einstein (HEAO-2) observatory

(Giacconi et al., 1979) the ﬁrst modern imaging X-ray telescope employed an MCP

detector. The same instrument design was reused after signiﬁcant further develop-

ments for the HRI on ROSAT (ROentgen SATellite) (Pfeffermann et al., 1987) and

the HRC (High Resolution Camera) of the Chandra X-ray observatory (Weisskopf

et al., 2002) that is still operable today 23 years after its launch. Furthermore, an

MCP detector was also used in the WFC (Wide Field Camera) of ROSAT (Barstow

et al., 1985).

Both detector types discussed here work in counting mode, meaning that they

both register individual photon events. While the proportional counter has an intrin-

sic (medium) energy resolution, MCP-based detectors usually only resolve energy

when used with a dispersive element like a grating and then exploiting their superior

position resolution. A second distinguishing feature is the need for a window for the

proportional counter to separate the gas volume while an MCP detector can be op-

erated open face. However, the gain in quantum detection efﬁciency by not having

transmission losses in a window for MCP detectors is always over-compensated by

the lower intrinsic quantum efﬁciency compared to a proportional counter.

Meanwhile, proportional counters as well as MCP detectors were almost com-

pletely replaced by their principal competitors in X-ray astronomy, namely silicon-

based detectors (Knoll, 2010). However, there are a few niches for which these

technologies are still developed: while MCP detectors are further optimized for the

UV and EUV where they can still be competitive to silicon technology depending

on the application and the requirements, position sensitive proportional counters are

applied recently to measure polarization in X-rays – a longstanding and scientiﬁ-

cally highly interesting topic that is now tackled by several missions.

This chapter presents in Section 2 the general concept of proportional counters,

their basic parameters, and some considerations on applying them in X-ray space

missions. In Section 3 the principle of imaging proportional counters and their ap-

plication in X-ray astronomy are discussed, including the relevance of micropattern

gas detectors for X-ray polarimetry. Section 4 explains the functionality of MCP

detectors and highlights applications in X-ray as well as UV and EUV astronomy.

Section 5 concludes with an outlook on the future prospects of both discussed de-

tector technologies.

4 Sebastian Diebold

2Proportional counters

The basic principle of a proportional counter is best introduced by looking at the

simple radial tube geometry as sketched in Figure 1. A thin wire is mounted coax-

ially in a cylindrical conductive tube, which is ﬁlled with the counting gas and

hermetically sealed. While the tube is on ground potential acting as cathode, the

insulated wire is connected to a positive high voltage via a load resistance Rland

forms the anode. When ionizing radiation enters the tube and generates ion pairs

in the gas, the high voltage accelerates the electrons towards the anode wire. For

the detection of soft X-rays a thin window is needed to allow transmission into the

sensitive detection volume.

cathode

VHV

X-ray

drift region

photoelectron

Ccharge-sens.

preamplifier

Vout

anode

wire multiplication region

Fig. 1 Sketch of a proportional counter in the simple radial tube geometry. An X-ray photon

interacting with the counting gas generates a photoelectron that loses its energy by ionizing gas

atoms. The generated electrons are drifted towards the central anode wire by an externally applied

high voltage. When the electrons enter the multiplication region around the anode wire they gain

enough energy to ionize neutral gas atoms and initiate a charge avalanche. The signal from the

anode wire is read out via a capacitively coupled charge-sensitive preampliﬁer.

The ﬁeld strength is increasing towards the anode wire, subdividing the gas vol-

ume in weak and strong electric ﬁeld regions, usually called drift region and multi-

plication or avalanche region, respectively. When the electrons gain enough kinetic

energy on a mean free path length, collisions with gas atoms can generate secondary

electrons in an avalanche process called gas multiplication (Knoll, 2010). For read-

ing out, the anode wire is capacitively coupled to a charge-sensitive preampliﬁer

that converts the collected charge to a voltage signal. If the applied high voltage and

thus the ﬁeld strength are set properly, the amplitude of the output signal is propor-

tional to the energy deposited by the incident ionizing particle or photon, allowing

counting and spectroscopy.

The complete detection process for soft X-rays in proportional counters is sum-

marized in a schematic overview in Figure 2. It should be noted that the photoelec-

tron is not necessarily carrying away most of the energy. A typical example is given

Proportional counters and microchannel plates 5

in Fraser (2009): a 6 keV X-ray photon is absorbed in a mixture of 90 % xenon and

10 % CH4. In this conﬁguration 99.83 % of the primary interactions are with the

xenon L-shell (mean energy 5.0 keV). These lead to the production of a 1.0 keV

photoelectron and a 4.2 keV L-ﬂuorescence photon (14 % probability) or a 3.4 keV

Auger electron (86 %) since the mean M-shell binding energy in xenon is 0.8 keV.

X-ray photon E

Transmission through

counter window

Secondary ionisation

Photoelectric absorption in

sensitive gas volume

pij = ai σij / ΣnΣm an σnm

Photoelectron

E – Eij

Auger electron

~Eij – 2Eij+1

Fluorescent photon

~Eij – Eij+1

Reabsorption Escape

Charge cloud

Drift and diffusion

Multiplication

Collection

Drift region

Avalanche region

ωij

eij

1 – ωij

1 – eij

Fig. 2 Schematic overview of the interaction process of soft X-rays in a proportional counter. If

the photon energy Eexceeds the binding energy Ei j of the jth shell of the ith component of the

counting gas mixture, pi j is the probability that a photoelectron is released from this shell. aiis the

fraction of the ith atomic component in the mixture and σi j are the respective photoelectric cross-

sections. The excited ion can relax either via the emission of an Auger electron or by emitting a

ﬂuorescent photon with the probability ωi j. The probability that the ﬂuorescent photon escapes the

sensitve counter volume is ei j.

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ProportionalcountersandmicrochannelplatesSebastianDiebold?AbstractDevelopedrightatthebeginningofthespaceageinthe1940s,thepro-portionalcounterwastherstdetectorusedinX-rayastronomyandstayeditsworkhorseforalmostfourdecades.Althoughtheprincipleofsuchadetectorseemstoberathersimple,overtimeitunderwentcon...

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