1.

INTRODUCTION

Detectors for receivers of LIDARs for Earth Atmosphere sounding of Clouds and Aerosols must achieve several basic requirements:

The first LIDAR echo profile of the Earth atmosphere was acquired by the LITE LIDAR¹ on-board of Discovery in 1994. Then several space LIDAR missions for Cloud and Aerosols sounding were launched (ICEsat1 2003², CALIOP 2006³). The detectors selected for these first missions were photomultiplier tubes (LITE 355 and 532 nm, CALIOP 532 nm) or Silicon Avalanche Photodiodes (LITE 1064 nm, ICEsat1/GLAS 532 nm, CALIOP 1064 nm).

Photo-multiplier tubes are still used but suffer from poor quantum efficiency, and generate operating constraints in space environment.

Silicon APD present larger manufacturability, easier operating conditions, but reduced gain and high excess noise factor.

ALADIN LIDAR on ADM Aeolus⁴ mission launched in 2018 for wind profiles measurements features an accumulation CCD manufactured by e2v UK.

Accumulation CCDs (ACCD) show good signal to noise performance thanks to an analogue accumulation of the return signal. But they request small sampling frequency and show reduced optical bandwidth. Moreover, the accumulation CCD suffer from transfer inefficiency on very weak signals and have shown in-flight occurrence of hot pixels issues⁵.

Progress in technologies and extended requirements in dynamic range, temporal resolution, spectral band, or bathymetry capability opened the way to new candidates such as InGaAs APD, Si-PM⁶ and MCP-PMT⁷. Trend to better vertical resolution needs fast temporal response.

InGaAs APDs allow to detect wavelengths up to 1.7 μm, as for the MERLIN⁸ mission aimed at methane sounding in the Earth atmosphere. They present however some limitations due to low gain and high excess noise factor.

Silicon Photo-Multipliers (Si-PM) are very promising, thanks to high gains and small excess noise factors as well as good industrial feasibility. They lack at covering a large optical bandwidth, and reveal significant Dark count rate degradation due to radiation environment in Space⁹. Thanks to significant efforts to reduce their radiation sensitivity, they should overcome this drawback in a near future.

Micro Channel Plate Photomultiplier Tube (MCP-PMT) emerge as good candidates for LIDAR missions due to a large set of photocathodes, with low dark count, high gain, high dynamic range and excellent temporal resolution.

MCP-PMT use a photocathode to convert incoming photons in electrons, then Micro Channel Plates under high electrical field are used as electron multiplier stage and one (in our case) or several anodes are collecting pulses.

We started our first characterization of a MCP-PMT in 2018, in the frame of a CNES and Airbus Defence & Space R&D joined activity. Photonis was selected as detector provider for its expertise and willing to progress in space applications. First steps included performance assessment and construction analysis. On the basis of the construction analysis, we kept on with an evaluation program which we planned to focus on a mechanical file with shock and vibrations, then characterization of the ageing process at the photocathode, proton radiation impacts and cathodoluminescence of the window.

This paper presents the main results of our test program carried out with Photonis and Airbus Defence & Space on a Hi QE, Hi linearity MCP-PMT.

2.

THE UV CHANNEL FOR MESCAL

This work was initially started to consolidate the French contribution to the MESCAL (Monitoring the Evolving State of Clouds and Aerosols Layers) LIDAR program managed by NASA in the frame of the A-CCP (Aerosols and Cloud-Convection Precipitation) study. The French Team promoted the 355 nm channel to ensure data correlation between CALIOP (for green and NIR) and EARTHCARE (for UV)¹⁰ as well as better speciation of aerosol types.

MESCAL is a type of up-graded follow-on of CALIOP, with the HSRL (High Spectral Resolution LIDAR) capability that allows to discriminate aerosols and molecules thanks to multi-wavelength measurements¹¹. MESCAL targets a 10 meters vertical resolution for aerosols and clouds and 1 meter resolution for bathymetry.

The 355 nm channel was finally abandoned in 2021, with no correlation with the results obtained so far on the Photonis MCP-PMT detectors which demonstrated good performance after the evaluation tests we present hereafter.

So the requirements and environment constraints were, as much as possible, based on MESCAL expected orbits and photon fluxes at the receiver input, schematically shown on Figure 1.

Figure 1.

Initial joined CNES-NASA LaRC architecture for MESCAL. The UV receiver was the French contribution. It was abandoned in 2021.

Three cases of illumination are illustrated below in Figures 2. The figures show three laser echo profiles (“useful flux” in photon/s along Y-axis) corresponding to the three different reception channels to be implemented on the UV receiver (these profiles may overlap over certain altitudes). Note that fluxes below 10⁵ photon/s were considered not relevant for the mission.

Figure 2(a).

LIDAR profiles for 355 nm wavelength, at 450 km altitude. Typical case for cloud free atmosphere with aerosols over vegetation. Two equivalent X-axis are provided, allowing the profiles to be read either versus altitude with respect to ground level (in kilometers), or time elapsed after reception of 40km-altitude echo (in microseconds).

Figure 2(b).

LIDAR profiles for 355 nm wavelength, at 450 km altitude. Typical case for atmosphere with aerosols and clouds.

Figure 2(c).

LIDAR profiles for 355 nm wavelength, at 450 km altitude. Worst case for atmosphere with presence of faint clouds in clean atmosphere over snow.

In addition to the laser echo, solar background also contributes to detector illumination (except during night time). Levels displayed in the figure are the maximum levels expected during daytime at detector-level.

3.

THE FAST TIMING MCP PMT

The Multi Channel Plate Photo Multiplier Tube (MCP-PMT) selected for the UV channel receiver is a Gateable Fast Timing MCP-PMT with 18mm open area (FT18-MCP-PMT) designed and manufactured by Photonis, with a low noise Hi-QE Blue photocathode on Quartz input window and single anode, with a typical QE spectrum shown on Figure 3.

Figure 3.

Quantum Efficiency (QE) of the Hi-QE blue photocathode of the Photonis FT18-MCP-PMT.

Its main performances and capability to handle photon counting as well as large signals due to ground echo as required for Space LIDAR for atmosphere sounding applications has been previously presented by Photonis^12,13.

Table 1 gives the main performance of the Hi QE, Hi- linearity model of MCP-PMT.

Table 1.

Specification of model series FT-18 MCP-PMT @ 25°C and nominal operation voltage (- 2300 V).

4.

RESULTS

4.2.

Lifetime tests

Reduction of the photocathode Quantum Efficiency (QE) during operating life is a known issue¹³.

Figure 4 shows QE evolution with charge extracted obtained on a commercial S20 Hi-QE Blue MCP-PMT at nominal MCP gain of 10⁵. The photocathode surface (18 mm diameter) was homogeneously illuminated with a 400 nm LED source.

Figure 4.

Relative change of quantum efficiency at 400 nm and gain vs total charge extracted from the anode, with extrapolated lifetime of about 4 C and 10 C for 20% and 50% QE drop, respectively¹³.

Ageing of the photocathode is usually due to electron-induced ion feedback and molecule desorption from the MCP and anode, causing degradation of the photocathode layer leading to a progressive loss of QE related to the quantity of electron charges generated along lifetime.

Expected Coulomb charges generated along a typical and worst-case mission, taking into account a necessary gating of the detector due to the large solar background during day measurements, were estimated from the atmosphere profiles to be about 6C, corresponding to 33% of QE drop (Fig.4).

Gating consists in a synchronized switching of the high voltage supplied between photocathode and micro channel plates, in order to limit the operating time window of the MCP-PMT during a 400 μs time interval of the laser period of 14 ms, when a useful signal is expected.

This gating strategy reduces the background-induced load on lifetime by the reverse of the duty ratio.

In order to maintain the initial QE over the mission duration of ten years, and/or allow higher operating gain, a Long Lifetime MCP design was tested.

The new design – that mainly consists in preventing the ion feedback at the photocathode was tested with results presented in Figure 5, showing no QE degradation at all measured wavelengths up to 19 C. Other performances remained as well unchanged. The test included 4.7 10¹⁰ gating cycles that were added to the test conditions within the last generated charge of 1.4 Coulombs.

Figure 5.

Relative change of quantum efficiency at 3 wavelengths and gain vs total charge extracted from the anode of the Long Lifetime FT18-MCP-PMT.

4.3.

Protons radiation

Impact of space radiations must be characterized first because radiations may change the basic performance of the detectors, and second because radiations may induce false events that corrupt the total photon counting.

Protons radiation was deliberately performed in conditions that will cover multiple LEO missions.

The total fluence depends on many parameters, including instrument shielding geometry, orbital parameters like altitude and inclination, and others.

The total in-flight proton fluence was estimated to be 1.8 10¹¹ p/cm2 for 60 MeV protons, assuming typical instrumental shielding equivalent to 4 mm aluminum, and a worst case 800 km orbit. This TNID would correspond to a total ionizing dose (TID) of 20 krad (Si) accumulated in the full mission lifetime of 10 years.

The proton radiation campaign was implemented by the Photonis team at KVI facility, in Groningen (The Netherlands).

Protons radiation of window substrates

A first phase consisted in measuring loss of transmission and photoluminescence (under excitation with 400 nm laser) in Photonis labs and then cathodoluminescence in CNES labs before and after radiation of several photocathode substrates: quartz substrates (FT18-MCP-PMT has a quartz based photocathode) and Borosilicate glass for comparison.

The Borosilicate glass was found to be impacted in the UV range with a 60 % loss of transmission at 350 nm after 5 10¹¹ p/cm².

The photoluminescence measurements after the radiation for the glass substrate samples radiated with highest dose of 5 10¹¹ p/cm² are shown on the Figure 6 (bottom) together with reference sample #21 (which did not undergo radiation). For quartz based substrates no change was found with very weak photoluminescence signal above 440 nm, that is in correlation with transmission measurements where no radiation-induced loss of transmission was observed.

Figure 6.

Photoluminescence- under 400 nm laser- measurements before (upper figure) and after the radiation (lower figure) of the glass substrate samples: non-irradiated ref sample Borosilicate (red dots); Borosilicate (purple); Hi QE UV quartz (black line) and Hi QE blue quartz (blue line).

On the other hand, a strong increase of photoluminescence signal (almost by 2 orders of magnitude above 850 nm) was found for the Borosilicate glass substrate. From transmission measurements, we know that after radiation this sample absorbs almost 50% of 405 nm light. From strong increase of photoluminescence in visible and infra-red part (and with photoluminescence shape to be about the same as before radiation) we can conclude that some part of absorbed laser photons recombine through the same radiative centers which were present in the glass before radiation. No additional strong photoluminescence peaks were observed in the photoluminescence spectrum.

Measurements of the cathodoluminescence of these glasses have then been performed at CNES to estimate the risk of parasitic light emission. The cathodoluminescence has been obtained using the electron beam of a scanning electron microscope.

Figure 7.

Cathodoluminescence spectra of quartz (a) and borosilicate glass (b) before (left) and after (right) 5 10¹¹ p+ (60 MeV)/cm² exposure. The electron beam energy to activate the cathodoluminescence was set to 15 keV.

The cathodoluminescence is fairly weak for quartz samples before and after radiation but the borosilicate glass shows a significant increase of the cathodoluminescence after irradiations, correlating with the photoluminescence results.

Protons radiation of MCP PMT

Radiation with 60 MeV protons up to a fluence of 5 10¹¹ p+/cm² was carried out at a flux of 1.45 10⁸ p/s/cm² on two high-linearity Hi-CE Hi-QE blue MCP-PMT.

At this high flux it was observed that single proton generates about 10 effective electrons with about 70 % originated from photocathode (~7 él/p+ from photocathode and ~3 él/p+ from MCP).

It is thought that the photocathode contribution is mainly due to the energy loss within the glass window (may be related to scintillation) and not because of a direct interaction between the protons and the photocathode layer.

Main performances were measured 7-8 weeks after radiations. QE and CE were found unchanged. No recognizable change was found in the gain (figure 8) and PHD shape.

Figure 8.

Gain vs MCP voltage for detectors #938130 (top) and #9452281 (bottom) before (left) and after (right) the proton radiation test.

Linearity of the MCP-PMT vs anode current is demonstrated on the Figure 9 with homogenous illumination of full photocathode area (appr. 18mm). Both detectors are high-linearity MCP-PMTs demonstrating extended linearity range. The 0.75 linearity threshold is reached at about 3 μA anode current and 0.5 threshold is at about 5 μA anode current for both MCP-PMT and with no recognizable change after the radiation.

Figure 9.

Linearity vs anode current for #938130 (top) and #9452281 (bottom) detectors before (left) and after (right) 60 MeV proton radiation test. The MCP gain is set to about 2.2 10⁵ for both detectors.

The temporal resolution - defined as the standard deviation of the Gaussian fitting the peak - as well as Pulse Signal FWHM before and after the radiation are very stable.

Small afterpulsing contribution of about 0.06%-0.08% is measured with no large change of afterpulsing after the radiation. The only recognizable difference was found in dark rates. The dark rate for both detectors were small (below 1 kHz), however small increase was found for both detectors after the radiation test: from 150 cps to 480 cps, and from 215 cps to 540 cps, respectively. It can be explained by small traces of radioactivity of the detector body, even 7-8 weeks after the radiation.

In flight false pulse occurence

The third radiation phase consisted in a set of four proton energies in the range [30 MeV -185 MeV] and various incident angles (0°, 45°, 60°) to quantify the cross section in order to predict occurrence of false pulses due to protons. The electrons/protons yield was registered for all couples of angle and energies.

The cross section σ was estimated using:

Where ne is the quantity of electrons induced by a proton, S is the detector area and θ the angle of incidence.

Table 3 gives the cross-sections for the 2 samples tested.

Table 3.

Calculated cross sections (cm2)

		Cross Sections Device 9383130 (cm2)	Cross Sections Device 1163170 (cm2)
Energy (MeV)	Angle (°)	Total	Photocathode	MCP	Total	Photocathode	MCP
30	0	3.88	3.82	0.05	5.23	5.01	0.22
	45	3.30	3.19	0.10	4.26	4.11	0.15
	60	2.00	2.00	0.00	2.61	2.61	0.00
60	0	9.68	7.11	2.57	15.43	10.46	4.97
	45	10.14	7.27	2.86	15.23	10.83	4.40
	60	12.71	8.92	3.78	21.27	13.92	7.35
100	0	7.74	5.64	2.10	12.58	8.43	4.14
	45	7.75	5.62	2.13	12.39	8.29	4.10
	60	8.04	5.80	2.24	12.97	8.68	4.29
184	0	6.92	5.51	1.40	10.66	7.93	2.73
	45	6.53	5.13	1.39	10.21	7.49	2.72
	60	6.39	4.98	1.41	9.96	7.24	2.72

The obtained cross sections were then used to calculate the mean probability of false event per day depending on the orbit. The two orbits taken as reference are given in Table 4.

Table 4.

Reference cases for false event occurrence

Orbit	Altitude	Type	Inclination	Duration
LEO	400 km	Circular	45°	10 years
LEO	800 km	Circular	98°	10 years

The radiative environment models used for false event forecasting are presented in Table 5.

Table 5.

Models used for simulations

Long-term models	Trapped particles	Protons	AP8-min
Galactic cosmic rays (GCR)	Protons	GCR-ISO solar min
Solar particles	Protons	ESP, 80%, 6.5 active years

Taking account a Weibull distribution where:

with W=35, S=2, E_th = 19.9MeV, σ_sat = 60cm², for the sample n°9383130, we obtained the false event rate per second for both orbits considered.

The false events per second were calculated using the formula:

Flux and fluences were calculated using OMERE¹⁴. Other calculations were implemented with Python3.7.

It is interesting to compare the false event rate per second to the photon flux dynamic range ([1 10⁵ – 1 10¹² including max ground echo] photons/s and to the false event rate due to thermal generation in the dark on ground that is about 300 – 500 event/s for our MCP-PMT at room temperature.

Table 6.

False event rate (event/s) in the SAA for trapped particles and at the poles for solar particles.

Events/s	400 km, 45°, 10 years	800 km, 98°, 10 years
0 mm	0 mm
Trapped particles Center of SAA	6 104	3.5 105
Trapped particles Edge of SAA	102	102
Solar particles		3 103

We observe that, with these worst case assumptions, the false event rate could reach more than 3 10⁴ events/s at the very center of the SAA.

Everywhere else, although the particles induced false event rate is larger than the on-ground dark induced false event rate, it remains under the dynamic range threshold of the useful signal.

Figure 10.

Event rate for trapped protons on a 400 km altitude circular orbit with 45° inclination, with 0mm equivalent aluminum shielding ad Weibull fit.

Figure 11.

Event rate for trapped protons on a 800 km altitude circular orbit with 98° inclination, with 0mm equivalent aluminum shielding ad Weibull fit.

Figure 12.

Event rate for solar protons on a 800 km altitude circular orbit with 98° inclination, with 0mm equivalent aluminum shielding ad Weibull fit.

5.

CONCLUSION

A novel design of Hi-QE Blue MCP-PMT has demonstrated promising stability of quantum efficiency up to 20 C of charge generation at the photocathode.

Proton radiation proved no significant changes for LEO missions over more than 10 years equivalent fluences with 4 mm Aluminum shielding.

Results obtained at this step improves TRL of the MCP-PMT technology and consolidates the position of Photonis MCP-PMTs for future space LIDAR missions.