Subsections

• 2.6.1 LWS readout electronics (integrating amplifiers)
  • 2.6.1.1 The ramps
• 2.6.2 Particle hits

2.6 The LWS Detectors

The LWS used photoconductive detectors which could provide background photon noise limited sensitivies when operated at the available temperatures in ISO. An understanding of the operation of these devices is important as any non-ideal characteristics will affect the data interpretation.

In a semiconductor at very low temperatures, the electrons fill the valence band. If the energy of a photon (wavelength ) incident on a photoconductor is greater than or equal to the energy gap ( ) between the valence band and the acceptor states, then an electron can be promoted to the acceptor states (and thus a hole is available for conductivity). Hence for an electron to be excited:

(2.10)

where is Planck's constant and is the speed of light. If the photon's energy is not as large as the energy gap then the photon cannot be absorbed; the material is transparent. This produces a limit on the longest wavelength detectable.

By applying an electric field across a small cuboid of photoconductive material the conduction electrons will move towards the positive potential thus creating a small current in the external circuit. By measuring this current the number of photons falling on the detector can be determined. In an intrinsic semiconductor, electrons are excited from the valence band to the conduction band. Doping a semiconductor produces smaller energy gaps in the detector and hence the wavelength range of the detector can be extended. Doped semiconductors are extrinsic and come in two types, and . In type semiconductors, a substituted impurity atom in the lattice structure donates an electron which is loosely bound to this positive impurity centre. For a uniformly doped device we therefore get energy levels just below the conduction band, called donor impurity levels. Bound electrons in these levels are promoted to the conduction band by the absorption of an appropriate long wavelength infrared (IR) photon because of the now much reduced band gap. For type semiconductors, the substituted impurity atom accepts an electron, thus effectively creating a hole loosely bound to the negative impurity centre. This creates energy levels just above the valence band called acceptor states. When an IR photon is absorbed, an electron from the valence band is promoted into this acceptor level but is still bound. This however leaves a free positive hole in the valence band which will, with the aid of an electric field across the detector, migrate towards the negative potential and thus produce a current. A good description of the physics and range of available photodetectors is given by Bratt 1977, [2]. The LWS photoconductors are all type. The long wavelength cut-off, as indicated in Equation 2.10, for Germanium doped with Gallium (Ge:Ga) detectors is at 115m. Until recently, Ge:Ga photodetectors represented the longest wavelength sensitive photodetectors available. However, it was found that by putting the detector under mechanical uniaxial stress the band gap could be effectively decreased, affording detection out to just beyond 200m (see for example Wang et al. 1987, [45]). Hence, the four longest wavelength channels in the LWS (LW2-LW5) are all stressed.

The LWS detector sub-system, shown in Figure 2.11, had three types of detectors: the shortest wavelength detector (43-50m), denoted SW1, was a germanium doped with beryllium detector, Ge:Be; detectors SW2, SW3, SW4, SW5 and LW1 were unstressed germanium doped with gallium, Ge:Ga(u), covering the total wavelength range of 50-110m; the long wavelength detectors, LW2-5, encompassed the range of 110-197m and were stressed germanium doped with gallium, Ge:Ga(s). Figure 2.7 showed the spectral response of these detectors. SW1 had a bias voltage of 600mV, SW2 had a bias of 200mV. The other Ge:Ga(u) detectors had a bias voltage of 150mV. LW2 and LW5 had the lowest bias voltages, at 40mV, and the LW3 and LW4 values were 60mV; this information is given in full in Table 2.3. Each detector was a 1mm sided cube.

Table 2.3: The ten detectors, their names, types, nominal wavelength ranges and bias voltages.

Detector	Type	Wavelength	In-orbit Bias
		Range [m]	Voltage [mV]
SW1	Ge:Be	43-50.5	600 (500 prior to rev.191)
SW2	Ge:Ga(u)	49.5-64	200 (150 prior to rev.191)
SW3	Ge:Ga(u)	57-70	150
SW4	Ge:Ga(u)	67-82	150
SW5	Ge:Ga(u)	76-93	150
LW1	Ge:Ga(u)	84-110	150
LW2	Ge:Ga(s)	103-128	40
LW3	Ge:Ga(s)	123-152	60
LW4	Ge:Ga(s)	142-171	60
LW5	Ge:Ga(s)	161-197	40

Figure 2.11: A schematic of the detectors and the illuminators.

Each detector was mounted in an integrating cavity to increase its absorption efficiency and is fed by a horn designed to couple efficiently with the radiation leaving the spherical mirror (8 of Figure 2.1).

The reduction in the energy band gap produced by a screw stressing the detectors, also makes them more sensitive to residual thermal energy, as the phonons can have enough energy to excite carriers into the acceptor states. This produces a larger dark current than in an unstressed detector. Under very low photon backgrounds, shot noise from this dark current limits the inherent sensitivity of these devices. The free carrier number density drops exponentially with temperature (the Boltzmann distribution is proportional to ), so cooling the detectors below 3K quickly reduces the number of thermally excited carriers and hence the dark current. It was found empirically (Church 1993, [8]) that by reducing the temperature of the stressed detectors to the lowest available temperature on board ISO, which was that of the helium tank at 1.8K, the dark current could be reduced to an acceptable level. However, it was found that when the unstressed detectors were operated at temperatures less than 2.5K the detectors would become unstable and spontaneously spike. This spiking increased in severity as the temperature was reduced to 1.8K, making the detectors unusable (Church 1993, [8]). This spontaneous spiking phenomenon is thought (Teitsworth, Westervelt & Haller 1983, [43]; Teitsworth & Westervelt 1986, [42]) to be caused by trapped space charge near the injecting contact causing local electric field breakdown within the detector with a subsequent `avalanche' of carriers. It is due to this spiking that the unstressed detectors (Ge:Be and Ge:Ga) are cooled only to 3.0K, and not to 1.8K as the stressed detectors, causing the mounting to be more complicated. At 3.0K the detectors can be operated nearly as well as at 1.8K because the dark current is only a factor of 3 higher (Church 1993, [8]). Operating the two types of detectors at different temperatures was achieved by mounting them on separate bars. The unstressed detectors were mounted on the upper bar, which acommodated a heater and a thermometer operating in a servo loop and it had a weak thermal link to the lower bar. The stressed detectors were attached to this lower bar, which was thermally shorted to the helium tank with a high-conductance copper strap.

2.6.1 LWS readout electronics (integrating amplifiers)

Each of the ten LWS detectors was read out by an integrating amplifier. This choice of readout was dictated by the very high impedance of the detectors under the ultra-low photon backgrounds experienced in flight, see Price 1993, [35] and Leeks 2000, [24] for more details. An integrating amplifier is, in principle, a resistor capacitor (RC) circuit. The readout circuit is shown in Figure 2.12. The voltage across the resistor in response to an input is given by:

(2.11)

where is the voltage supply, the time of build up of the voltage on the capacitor, is the resistance and is the capacitance. For then, expanding the exponential to first order, Equation 2.11 becomes:

(2.12)

and differentiating with respect to gives:

(2.13)

so by measuring the voltage build up in a known capacitor, the current can be recovered. This is the essence of the Infrared Labs. JF4 integrating amplifier as used in the LWS.

For a photoconductor under low photon flux, the photon noise is negligible so the dominant noise processes are Generation-Recombination (GR) noise [rms noise current, ] and electron shot noise [rms noise current, ] for a post detection bandwidth of 1Hz. Here is the total current flowing in the detector circuit, is the electronic charge and is the photoconductive gain (see Bratt 1977, [2] for a detailed description). Since these two mechanisms are uncorrelated the total noise current is given by their quadratic sum:

(2.14)

where is known as the dark current (the current flowing in the absence of photons).

For the LWS the average photoconductive gain, , is 0.66 (see Table 4 of Swinyard et al. 2000, [41] for the individual values of ), giving . The responsivity for the detector circuit, , gives the current, , which flows in the detector when illuminated by an infrared signal of power , such that:

(2.15)

where is the Responsive Quantum Efficiency, is Planck's constant, is the speed of light and is the charge on one electron. The NEP () is then:

(2.16)

Hence for dark currents of 100 electrons/s and an product of 0.1, an NEP of 5 10WHz at 100m is achieved.

Note that the read noise of the integrating amplifier was measured to be 10 electrons regardless of the integration time, which is significantly less than the dark current.

The integration amplifier therefore gives a photon-noise-limited performance. It is like a trans-impedance amplifier but with an infinite feedback resistance so that there is no Johnson noise from the feedback resistor to limit the performance of the detector circuit. This made the integrating amplifier very suitable for the LWS.

Figure 2.12: The integrating amplifier readout circuit for an LWS detector. are source resistors for the JFETs and is the heater resistor. N is the integrating node.

2.6.1.1 The ramps

The integrating amplifiers used typically had a gain of 0.9 and a capacitance of 7.5pF. The photons incident at a detector produced a flow of charge which accumulated at the input capacitor. The voltage produced as this capacitor charges was non-destructively read out 88 times per second (sampled every 11.4ms) through the JFET#2 (see Figure 2.12). After an appropriate time this capacitor was discharged by applying a reset pulse through reset JFET#1 at the gate of JFET#2 and also applying a compensation pulse which is adjusted to neutralise any residual charge left from the reset. After the reset the build up of voltage began again. JFET#3 balances out the offset voltage of JFET#1 so that the voltage at output #6 represents that measured at the gate of JFET#2 alone.

Two different integration times (0.5s or 0.25s) were available with the LWS; for strong signals the output was less noisy and more non-linear hence the shorter integration time was more suitable. Ideally, for very strong sources, the integration time would have been reduced even further. However, the dead-time (the time taken for the signal to settle down after a reset) was 50ms and so reducing the integration time to less than 250ms would have lowered the overall on-source efficiency and given very few data points in the ramps for statistical evaluation.

The resetting of the circuit produces a series of `ramps', where one ramp was the voltage build up between two resets representing the signal collected during the interval. Figure 2.13 shows twelve such one quarter second ramps for SW3 (22 data points per ramp; half-second ramps have 44 points). One full grating spectrum has typically 2500 ramps. The build up of charge caused a drop in the effective bias voltage (this is called de-biasing) which in turn caused a change in the responsivity. This effect was quite large in strong sources and it produced a non-linear response to the input signal. Effectively for the same increase in input the output does not increase by as much so the ramps are curved.

Figure 2.13: This figure shows 12 one quarter second ramps from an observation of W28A2 on detector SW3. The data points are shown by the diamonds.

The ramps represent the incoming photon flux from the astrophysical object observed and are converted to photocurrents in the second stage of the automatic data processing (see Section 4.1).

2.6.2 Particle hits

ISO was subjected to particle hits, while in orbit about the Earth, from energetic protons and high energy electrons. The effect of these charged particle impacts on a detector is to generate electron hole pairs from which the intrinsically generated electrons neutralise the compensation donor impurities and thus increase the mobility and hence responsivity. For laboratory dosage of high energy gammas (60keV, 1000 s) it was possible to saturate the responsivity after 100s, with typical observed responsivity changes between factors of 10 and 50. Clearly any gain change of the detectors needed to be carefully monitored if any sense was to be made of the astronomical data. The most dramatic effect of particle hits was when passing through the Van Allen belts.

Figure 2.14: Photocurrents from SW4 of an illuminator before (upper data) and after (lower) a bias boost.

The responsivity of the detectors increased due to the particle hits: to re-normalise the responsivity, the bias current was increased to beyond the breakdown voltage for each detector. This bias boosting causes impact ionisation of the neutral impurities and therefore largely restores the pre-irradiation values of donor and acceptor levels. Operationally, bias boosting was applied on exit from the Van Allen belts. Figure 2.14 shows the reduction of the responsivity of LW4 after a second bias boost which was performed about half way through the 24 hour orbit. The data are from an illuminator flash before and after a bias boost half way through an orbit.

Figure 2.15: Three moderate strength integration ramps are shown; the middle one shows a glitch.

This boost was required to restore the responsivity of the detectors which also increased during the orbit as the satellite was constantly bombarded by galactic cosmic rays (H and He nuclei). However the dark currents were not affected by the cosmic rays, they remained constant during an orbit (Swinyard et al. 2000, [41]). These cosmic ray hits, about one every eight seconds, also caused the readout signal to jump (this is termed a glitch) as shown in Figure 2.15. It was found that a glitch changed the responsivity for the rest of that integration and the following ones, hence these data could not be used. The part of the integration before the glitch could, however, still be used to find the photocurrent caused by the incident photons. The longer the individual integrations, the greater the number of resets required to recover from a glitch. To reduce the number of unusable integrations, and hence the amount of observing time wasted, the length of them was reduced, from the pre-flight specification of two seconds, to one half-second (one quarter-second for strong sources, see also Section 5.7). It was then found that only two integrations after the glitched one were affected and should not be used. This still presented the high possibility of there being no good data for a particular wavelength if they were collected consecutively, hence `fast scanning' became the standard. This was where just one integration per wavelength interval was recorded and the grating was scanned to cover the required wavelength range, producing one `scan'. This was repeated to produce several scans which could be co-added.

The change in responsivity between the bias boosts was monitored by the use of IR illuminators. The five illuminators (labelled 1 to 5 in Figure 2.11), which are located in front of the detectors, output a known IR signal which is used to monitor the responsivity changes. The data of these illuminators are used to correct for the drift in responsivity with time during an orbit and an individual observation as discussed in Section 4.1.