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Some of your more Frequently Asked Questions regarding Photonis Scientific detectors.




Q.- How do I store MCPs and Channeltrons?

A.- The nature of the materials used in manufacturing microchannel plates and Channeltron detectors are such that certain precautions are required in order to preserve the high performance characteristics of these detectors.

The secondary electron emissive surface is composed primarily of an alkali doped silica layer, and as a result, these materials are very hydroscopic. Special care and handling is necessary in order to prevent the absorption of water vapour and ensure optimal detector performance.

Typically, Microchannel Plates (MCPs) are placed inside a metal shipping can and the can is then heat sealed in a laminated foil bag. The bag is back-filled with dry nitrogen and then evacuated. This container is suitable for the shipping period only. Upon receipt of the MCP, the device should be removed from its shipping case and the foil bag and stored in a vacuum. If the MCP has been shipped in a metal can, the can should be opened in a clean room environment and the top metal "spider" component, which holds the MCP in place, should be removed prior to placing the MCP in the can in storage. If vacuum storage is unavailable, storage in dry nitrogen is a suitable alternative.

Some Channeltron detectors are shipped in a hermetically sealed bag. Others are shipped in a plastic pre-form case. Both are suitable for short shelf storage periods (typically six months or less). Storage in a vacuum or dry nitrogen is recommended.

Microchannel plates and Channeltron detectors can be degraded by exposure to various types of hydrocarbon materials which raise the work function of the surface, causing gain degradation. Operation in a clean vacuum environment of 10 -5 torr or better is necessary in order to ensure the long-life characteristics of these devices.

Q.- What is the life time of a MCP?

A.- Photonis has pioneered a family of long-life glasses that are used exclusively in Photonis Long-Life Microchannel Plates MCPs). These glass types have been specifically engineered to be very stable, providing extended operational life.

The lifetime of a microchannel plate ultimately determines the useful working time of the device in which it is placed. Figure 1 shows the superior gain stability of Long-Life MCPs as compared to conventional MCPs. Decreased lifetime for microchannel plates usually involves changes in the first strike conversion efficiency when a primary event, an ion or photon, strikes the channel wall on the input side. if the conversion efficiency decreases over time, then the useful lifetime of the multiplier will also be compromised. The intrinsic secondary electron yield loss within the channel caused by contamination or radiolytic damage to the inside of the channel walls may also decrease lifetime. Care should be taken to prevent exposure to high concentrations of halogens and hydrocarbons. Photonis's long-life glass composition has been specifically formulated to minimise the effects of these two loss mechanisms.

Graph of ageing

Fig 1

Figure 2 demonstrates the typical gain degradation of a Long-Life Microchannel Plate as a function of extracted output charge in terms of coulombs per square centimetre. After an initial burn-in period, in which the detector gain changes as a result of degassing residual gas molecules from the inside of the channel, detector performance is very stable over a large amount of extracted output charge.

Graph of ageing

Fig 2

Typically, approximately 40 coulombs per square centimetre can be extracted from a microchannel plate in a clean working environment without significant gain degradation. Other glass systems typically will produce a stable gain operating period of about one to ten coulombs per square centimetre.

Q.- What is the time response of MCPs and channeltrons?

A.- A very important performance parameter for electron multipliers is the temporal or time response of the detector. The detection time is essentially proportional to the aspect ratio - the thickness of the Microchannel Plate (MCP). Since these devices have very short path lengths, they tend to have very short response times. And the high internal electric field within the channel guarantees that the electron multiplication process happens very quickly.

Typical results from this type of a design are that the transit time of the pulse within a channel is very short, typically one nanosecond. Pulse spreading is minimised, resulting in narrow pulse widths of about one nanosecond.

Microchannel plates are used in a number of very time-dependent applications. Figure 3 shows some typical results of a small pore microchannel plate and the resultant temporal response after having stimulated the detector with a single ion event. The amount of propagation time (amplification time) can then be measured and recorded using a high speed digital oscilloscope. In Figure 4, a 0.51 nanosecond pulse width and 0.34 nanosecond rise time were achieved with a 25mm format matched set of 5~m pore microchannel plates.

Scope screenshot

Fig 3

Table 1 details temporal responses typical of a variety of detector configurations.

Table 1
Detector
Type
Detector
Voltage
Pulse
Width
Rise Time
(Tr Actual)
5um Pore (60:1 L/D) 2200v 0.5ns 340ps
10um Chevron 1870v 1.8ns 283ps
25um Chevron 2000v 2.5ns 640ps
Typical channeltron 3000v 18-20ns 3000-5000ps
Descrete Dynode Multiplier 2000-3000v 30-50ns 6000-10000ps

Q.- What is the spatial resolution of MCPs?

A.- An important performance parameter of the Microchannel Plate (MCP) is spatial resolution. The ability of a microchannel plate to resolve spatially, two adjacent events is called the limiting resolution.

The limiting resolution of a micro channel plate is ultimately dictated by the channel pitch, sometimes referred to as the centre-to-centre spacing. MCPs are fabricated with channel pitches ranging from 6 to 32 microns.

Limiting resolution is characterised in terms of line pairs per millimetre. Figure 4 indicates the relationship between channel pitch and limiting resolution.

Resolution graph

Fig 4

The limiting resolution of a detector system can be affected by parameters other than those dictated by the microchannel plate. The spacing and field strength between the output side of the microchannel plate and the readout device must be optimised in order to preserve the maximum resolution of the detector. In addition, the input events to be imaged must be properly focused on the detector input side.

The spatial resolution of an MCP detector may be optimised through the use of small pore microchannel plates and deep output electrode endspoiling.

Q.- What is the detection efficiency of MCPs?

A.- The detection efficiency of a Microchannel Plate (MCP) is an important performance parameter. In many applications, the abundance of the signals to be detected is quite small. And, therefore, it is important that every photon and every ion be detected and added to the signal statistics.

Some of the features that can affect the detection efficiency of a microchannel plate are the:

Microchannel plates can be customised in a number of ways to enhance the detection efficiency or, in some cases, to lower the detection efficiency for a particular desired or undesired species that might be present in the instrument 0 the experiment. Various types of secondary electron emissive coatings, such as cesium iodide, magnesium fluoride, magnesium oxide, copper iodide, and gold, can be used to significantly enhance the detection efficiency for various charged particles and electromagnetic radiation.

In some detector applications, the use of a suppression grid is also helpful in enhancing the detection efficiency.

Figure 5 shows the detection efficiencies typically achieved for various common input phenomena.

Detection efficiency graph

Fig 5

Q.- What is the dynamic range of MCPs?

A.- An important performance feature of a Microchannel Plate (MCP) detector is the dynamic range. The dynamic range is the range of detectable signal level over which the MCP linearly amplifies a signal.

The dynamic range of a microchannel plate or any electron multiplier when detecting extremely low signal levels is limited by the dark current or the background noise inherent in the multiplier. There are a number of ways to minimise the effects of the dark current or dark count, including the use of some new specialised low noise glass formulations which have recently been developed.

The background count rate of a microchannel plate is limited ultimately by the cosmic ray background found here on earth. The background count rate of the glass itself is limited by the amount of radioactive decay of trace impurities found in the glass. The glasses used to manufacture Photonis electron multipliers have been specially formulated to produce low background noise.

The upper limit of dynamic range is ultimately limited by charge saturation effects within the multiplier itself.

There are a number of ways to raise the high end limit of the dynamic range of a microchannel plate based detector. The most effective way is to simply lower the resistance of the individual channel. Each channel can be considered an effective resistor capacitor network -- by simply lowering the resistance of the channel, the "dead time" of the channel is diminished proportionally to the change in the resistance.

The Photonis long-Life tm glass composition is specifically tailored to be able to produce very low resistance channels. Low resistance microchannel plates are called Extended Dynamic Range MCPs or EDR plates.

Another effective means of increasing the upper count rate of a microchannel plate is to increase the channel redundancy per unit area. Microchannel plates used around the world are manufactured with pores as large as 100 microns and now as small as five microns.

By effectively increasing the number of channels covering the same area of a microchannel plate, the number of missed events caused by a second event entering the specific channel within the dead time of the channel is greatly diminished.

Figure 6 is an example of going from a 25 micron pore channel down to as small as a five micron pore channel. If an electron were to enter the 25 micron channel during the time frame it took for that channel to regenerate, a second event coming in would be totally lost. 1£, however, the larger area (25~m) were replaced by a number of smaller ones (5um), only the individual 5 micron channel would be dead for that time period, and a second event entering another channel displaced by as little as five microns would be detected and amplified. Photonis now manufactures microchannel plates with pore sizes as small as five microns.

Channel density schematic

Fig 6

Figure 7 defines the best MCP to optimise dynamic range performance for various input signal levels

Schematic showing best type for various input levels

Fig 7

To summarise, dynamic range can be increased on both sides of the operating range of a microchannel plate. The threshold for detecting low signal levels can be enhanced by going to a lower noise glass composition, while high count rate or high current application can be enhanced by simply going to low resistance microchannel plates and smaller pore MCPs.

Q.- How do I achieve a saturated Pulse Height Distnbution (PHD) when pulse counting in Channeltrons or Microchannelpiates?

A.- PHD is the distribution of the amplitude of the output pulses from a CEM or MCP. Single MCPs and Channeltrons designed for analogue operation generally produce a negative exponential PHD curve as shown below.

Graph of pulse height distribution

Fig 8

If the output pulses can have a wide range of amplitudes as shown in this negative exponential distribution then it would be expected that in a pulse counting application the smaller pulses could be confused with noise and the measured count rates could be sensitive to galn shifts. Ideally the output pulses should be all of the same amplitude for good pulse counting. This ideal is approximated in CEMs or MCPs by ensuring that they operate in the space charge saturation mode. In this mode there is sufficient gain such that saturation is caused by space charge and the pulse height distribution becomes peaked. Or more accurately a quasi-gaussian PHD, as shown below, in which the output pulses are all very nearly the same amplitude.

Graph of pulse height distribution

Fig 9

In practice space charge saturation is achieved near the channel output when at high gains the space charge density electrostatic repulsion reduces the kinetic energy of the electrons hitting the channel walls and so less secondary electrons are produced.Less secondary electrons at the rear of the channel in turn decrease the space charge allowing the electrons to produce more secondaries and so a dynamic equilibrium is achieved. Although the high gains necessary for a peaked output PHD are achieved in Channeltrons typically MCPs have to be arranged in a Chevron configuration. As is shown in the figure the PHD is generally characterised by the ratio of its full-width-at-half- maximum(FWHM) of the peak to the peak value in the pulse height distribution.It is normally expressed as a percentage. The narrower the PHD the smaller the FWHM and all the output pulses are approaching the same value. Typically pulse counting CEMs yield a FWHM of less than 75% with some as low as 20%. In general the larger the length to diameter ratio of the device, the narrower the PHD. Similarly as shown in the table below for MCPs a z-stack(threestage) assembly of high length to pore diameter ratio achieves a FWHM of less than 60%.

Table 2
Configuration L/D Ratio Maximum
Voltage
Gain Pulse Height
Single MCP 40:1 1000 >5(10)3 Neg.Exp.
60:1 1300 >5(10)4 Neg.Exp.
Chevron Assembly 40:1 2000 >5(10)6 <150%
60:1 2600 >2(10)7 <75%
Z-Stack Assembly 40:1 3000 >5(10)7 <120%
60:1 3900 >2(10)8 <60%

Q.-What is the maximum operating voltage of a microchannelplate?

A.-The operating voltage is defined as a potential difference between the input side and output side of a microchannelplate. It is dependent on the length-to-diameter ratio(L/D) of a microchannelplate. The rule of thumb is that multiplying the L/D ratio by 25 will give the maximum operating voltage. For example, the maximum operating voltage for a 40:1 L/D MCP is Vmax=40x25=1,000 volts.

Q.- Why should I use MCPs with a matched resistance in the Chevron configuration?

A.- In order to butt mount two MCPs in Chevron configuration, their resistances should be matched. If not the voltage will divide unevenly on each MCP due to their different resistances. This will result in a different gain in each MCP and possibly inadvertently exceeding the operating voltage across one of the MCPs. Users have separated two unmatched resistance MCPs by placing a centre ring tab in between them.This results in additional cost in the form of a voltage divider or an extra power supply. Also the gain and output resolution are likely to be not as good as a butt mounted configuration. However some users have narrowed the pulse height distribution by applying an accelerating voltage of hundreds of volts in the region between the two plates -This requires two electrode rings insulated from each other on the inner MCPs.

Q.-How can I clean MCPs?

A.- You will need a stir plate, two 2000ml heavy duty beakers, plate holders, an ultrasonic cleaner,dessicator, electronic grade or better isopropyl alcohol, magnetic stir bars.

  1. Fill two beakers with isopropyl alcohol and place on hot plates.
  2. Put plates into first beaker and stir isopropyl alcohol and bring to the boil. Let boil for 5 minutes.
  3. Remove boiling isopropyl alcohol beaker from the hot plate and put into the ultrasonic for one minute.
  4. Put plates into second beaker and stir isopropyl alcohol and bring to the boil. Let boil for 5 minutes.
  5. Repeat procedure 3.
  6. Use same beaker of boiling isopropyl alcohol stir and boil for 5 minutes.
  7. Vacuum dessicate dry plates for half an hour at 100 degrees Centigrade.

If all you have is single particle contamination this can just be removed withan ionised dry nitrogen gun without going through all the above procedure.

Q.-Is the container used to ship my MCP appropriate for long term storage?

A.- No. The MCP shipping containers are not suitable for storage periods exceeding delivery. Upon delivery the MCP should be ideally transferred to an oil free vacuum chamber. A dry box that utilises an inert gas such as argon or nitrogen is also suitable as the MCPs are very hydroscopic and if left in air any water vapour will be absorbed which can cause expansion and stress cracking.

Q.-What is the difference between Detector quality, Imaging quality and Premium quality MCPs apart from their price?

A.- Any batch of MCPS produced will have a certain quantity with a very small number of physical defects such as areas of missing channel walls or holes. Out of this batch Detector quality MCPs will be those selected that do have some of these defects that would mean that if the MCP was used in an imaging application at its highest spatial resolution the image could have a few small blemishes in it. However for signal detection applications such as used in time-of-flight mass spectrometry, residual gas analysers, point source detectors and even multi-electrode detectors the Detector Quality will perform to the highest specification and so are the preferred choice. Image quality MCPs are by definition those selected without image impairing defects such that they can be used in conjunction with an appropriate readout to provide an intensified high spatial resolution image. So that Image quality MCPs are found in applications such as second generation image intensifier tubes, ultra-fast cathode ray tubes and analytical techniques such as imaging mass spectrometry and VUV spectroscopy. Although Image quality will produce images completely suitable for such applications there are a further set of applications such as streak cameras, third generation night vision systems and space applications that demand the ultimate imaging capability and it is this area for which Premium quality MCPs are intended.

Q.- What are the advantages of small pore MCPs?

A.- We are increasingly supplying smaller pore Photonis microchannelplates. Photonis's five micron pore (6 micron center-to-centre) MCPs have been available for some time in the 18mm active diameter MCPs and have recently become available in 25mm active diameter MCPs. Users of these five micron MCPs have demonstrated greater than 71 line-pairs per millimetre (14 micron) resolution in image intensifiers. These MCPs also have the advantage of a faster timing response compared to larger pore MCPs because of their reduced thickness, yielding sharper pulses with faster rise times,as well as faster recovery from large signal inputs due to more channels per area, and a larger open area for increased sensitivity.

Q.- What are the advantages of the 60:1 L/D MCPs?

A.- We are increasingly supplying the Photonis Advanced Performance detector assemblies with the 60:1 Length to Pore Diameter(L/D) ratio. These thicker plates have a number of advantages over the 40:1 (L/D) MCPs that have been traditionally used in detector assemblies:

Q.- Do Photonis sell very low-noise Microchannelplates ?

A.-Decreasing the background event rate in microchannelplates is an effective method of increasing the sensitivity of astrophysical instruments(1) Photonis has developed a proprietary low noise glass formulation which reduces the background rate to the cosmic ray level. Recent experiments have shown a Z-stack configuration background events measured in the laboratory of 0.06 events cm-2 sec-1. SUP>(2) Custom sized microchannelplates manufactured with Photonis's exclusive Low-noise Long- Life™(L3N) Glass formulation are now available.

  1. O.H.W.Siegmund, D. Marsh, J. Stock, G.Gaines, SPIE Vol. 1743 EUV, X-Ray and Gamma-Ray instrumentation for Astronomy III (1992)p281
  2. J. Edelstein, personal communication, University of California Berkeley, Center for EUV Astrophysics, December, 1995.

Q. What is the best detector I can buy for TOFMS applications?

A.- Recently Photonis has announced a new Microchannelplate Time-of-flight mass spectrometer detector which has a number of unique features, including easy replacement of MCPs using a rugged reusable cartridge, high aspect ratio MCPs providing gains in excess of 107 and 5 micron diameter pores offering the highest channel density available for high dynamic range. A low profile matched impedance conical anode provides sub-nanosecond pulse widths.

Q. My laboratory buys lots of Channeltrons can I have a price discount?

A.- Kore has introduced Photonis's Labsense® program designed to meet the special needs of laboratories that need to regularly replace Channeltron® detectors in their mass spectrometers. Photonis manufactures replacement detectors for virtually every commercial mass spectrometer and by enrolling in the Labsense® program these are available ex-stock from Kore at considerable savings off standard list prices. There is also a no-nonsense performance guarantee and an easy risk-free trial.


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Last updated:24 October 2009 16:19

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