Multichannel analyzers

Almost every experimental physicist needs a multi channel analyzer (MCA) – many of them just do not know it. This sentence is no provocative statement but plain daily experience.

MCA are on the market for over 60 years. The first MCA were monstrous power-hungry vacuum tube units having only few channels, a poor stability and high price. Performance specifications of early MCA could be measured by their volume in m³ and power consumption in kW. Early MCA could only be operated by specially trained technicians who needed at least one weekly service-day for tube testing, equilibration and exchange. With the advent of transistorised MCA in 1959 a rapid development started which brought small, fast and cheap units with more than 256 channels. First MCA models were completely controlled by its integrated hardware, however, soon there was implementation of a central processor and program memory. Thanks to these computer features MCA provided increasingly complex display and handling functions, even simple spectrum analysis functions were implemented.

Thanks to even more powerful Application Specific Integrated Circuits (ASIC) one finds now complete measuring and analysis systems for specific applications which are designed around an MCA. There are for example very powerful X-Ray Fluorescence systems, Electron Excitation Spectrometers (µ-sonde), Optical Multichannel Analyzers (OMAs), or systems where very many detectors and MCA can be operated for multi-parameter measurement (e.g. CAMAC and developments derived thereof). The latter systems can measure very many parameters simultaneously in one single experiment and thus provide results fast and cheap. Events from multi-parameter measurements are mostly stored in LIST mode where each event is stored individually together with a high-resolution time stamp. In this way one can sort data according to defined criteria after the experiment and select whichever detail is to be surveyed. Even details that were not considered when the experiment was made can later be sorted and analysed from LIST mode data.

The desriptions below relates to “classical” MCA which are employed for nuclear spectrometry. Some of these units can be enhanced with optional hardware and software for multi-parameter measurements of multi-parameter applications with several detectors.

The MCA principle

The MCA in pulse height analysis mode (PHA) is a counter which registers in integer display the frequency with which numbers are provided by an analog-to-digital converter (ADC). The maximum digital number to be dealt with is defined by the number of storage elements provided. Each storage element is called a “channel” and each channel stores the frequency of occurence of the number which is identical to the channel number. For example, in a spectrum having 1024 channels, the frequency of occurence of the number 1024 is stored in the highest (last) channel and the frequency of occurence of the number 123 is stored in the 123^th channel. Modern MCA may provide up to 32768 (32k) or even 64k channels; however, for almost all applications 8192 (8k) channels are enough. Only prompt gamma-ray spectrometry where the energy range goes up to over 10 MeV needs 16k spectrum length. The biggest number of occurences that can be stored in a channel is defined by its memory size. Normally one will have 4 bytes for one channel from which 31 bits are used for counting up to 2³¹-1; however, some MCA use only 23 bits for counting and the rest for other purposes like region-of-interest marking.

The two most frequently used external units that convert a signal into a number for registration in the MCA are the Analog-to-Digital Converter (ADC) and the Time-to-Digital Converter (TDC) which often is a Time-to-Analog Converter with subsequent ADC. The ADC converts a continuously varying voltage (sampling mode) or the pulseheight of an electric signal into a digital number which is linearily related to either the height of the voltage/pulse or to the time difference between a start and a stop signal. The input signal is measured with some type of detector and then electronically processed so that it represents well the property to be surveyed. In most applications a linear relationship between signal height and surveyed property is preferred.

There are other operation modes of MCA in addition to PHA. In Multi-Channel-Scaling mode (MCS) there is no conversion of pulseheight but all incoming signals are rather counted in the same channel. After a predefined time, called the dwell-time, the channel number is advanced by one and all incoming signals are counted in the next channel, etc… In this way one measures the time dependent countrate that comes from the detector and one gets for example a direct display of the decay of a radioactive sample. In List Mode one stores the pulse-height of each event together with a high-resolution time-stamp onto a very large mass storage device. The large data set is sorted after the experiment according to defined criteria. Multiple sorting and analysis passes are possible, even after years. One can for example follow the signal height as a function of time (transient recorder) or search for correlations in time between certain events (coincidences). List mode is normally applied in multi-parameter measurements employing several detectors.

 

Application examples for MCA

In the following the most important applications of MCA are presented.

Energy-dispersive nuclear spectrometry
In this classical MCA application radiation from a source is measured with a suitable detector (gamma-rays, alpha particles, X-rays, fission products, or products from induced nuclear reactions). The energy or other property of each event is converted to a digital number which is stored in the spectrum in PHA mode. If certain energies are more frequently registered than others one finds peaks in the spectrum which sit on top of a more continuous background distribution. The position of a peak in channels (=energy) and the net area of the peak (=total frequency) can then be analyzed with appropriate mathematical procedures.

Time-spectrometry
For the measurement of time differences between events one uses a Time-to-Digital Converter (TDC) or a Time-to-Amplitude Converter (TAC) followed by an ADC. The TDC provides an output signal whose height is linearily proportional to the time difference between a START and a STOP pulse provided to the TDC. This TDC number is stored in PHA mode in the MCA. In this way one measures a spectrum in which the channel number is equivalent to a time difference.

This simple method for measurement of time differences is applied to determine distances, cable length or the jitter in electronic circuits. A more recent and very powerful application of time spectrometry is Time-Of-Flight Mass Spectrometry (TOF-MS) where molecules are cracked by a suitable excitation, charged fragments are accelerated in an electric field and the flighttime of fragments is measured with micro-channel plates as stop detector. The mass of the fragment which is proportional to the flighttime can be determined from the spectrum. Using modern equipment one can determine masses in the range 1 amu < mass < 150000 amu in one measurement; resolution up to mass ~1000 is better than one amu!

Measurement of particle size
For measurement of particle sizes the objects pass in high dilution (one single particle at a time) a broad beam of coherent laser light. A very sensitive detector measures the intensity distribution of scattered light as a function of scattering angle and registers the light intensity (signal height) in PHA mode in an MCA. With the aid of calibration measurements one can convert the light intensities measured under a certain angle into particle sizes. The range of particle sizes to be measured with this method is between 50 nm and 50 µm, depending on the wavelength of the laserlight employed.

Searching/ Scanning
When one searches the location of an object or event then the searching range is scanned with a focussing detector and the MCA is operated in MCS mode. The advance of the channel number (dwell) is synchronized with the advance of the detector, i.e. each channel relates to a specific focus point of the detector. A second MCA then measures one PHA spectrum from each focus. In this way one can determine the local dependence of certain events or the energy distribution found in certain locations. Such measurements are applied in X-ray astronomy or for the assay of angular distributions of products from kinematic or nuclear reactions.

The compilation of experimental applications of MCA is almost endless. Almost each property can be prepared in such a way through appropriate selection of the measuring set-up, the detector and electronic signal conditioning that it can be quantified most effectively with an MCA. Combinations of methods most often find more complex answers to questions.
We will be glad to support you in finding appropriate set-up and equipment to solve your experimental task.

Status of MCA development

Following the enhanced integration of computer power into MCA different types of instruments were developed out of which three types have maintained: Stand-alone units, NIM-technology and Plug-on-MCA for scintillation detectors. For some time one could find MCA as PC plug-in cards; however, these units are hardly built any more because technical development of PC is outpacing development of new MCA.

Stand-Alone units are direct follow-ups of earlier MCA. They employ programmable IC with measuring programs and very often also analysis programs to solve all tasks for specific applications. Most units provide additional functions for marking, classifying or norming of measured data as well as for the control of complicated experimental sequences. Almost all stand-alone MCA can store very large numbers of spectra and/or they have normed interfaces that connect into external computers for data transfer. Many units have an integrated amplifier and ADC as well a power supply for the detector and other attached units. There are small portable units for “in-situ” applications which run for up to 8 hours from the battery. Recent units have digital amplifiers/shapers and very fast flash-ADC which enable very high counting rates with good spectral specifications.
Stand-alone units are very well suitable for all measuring tasks, however, they do normally not provide the possibility to integrate free programming of data analysis.

MCA in NIM-Technology are small stand-alone­ units which miss all components that are anyway available in other associated components; in this way NIM-MCA are cheap and attractive. They do not provide a monitor screen or keyboard, so they require to have a Personal Computer (PC) for set-up and control, as well as a NIM-crate for power supply to the NIM-modules. A set-up and display software is running in the PC which serves the preparation of the NIM-MCA. One can inspect current data during measurement on the PC and modify parameters accordingly. One can also disconnect the PC during a running measurement and use it for other purposes. The configured NIM-MCA will conduct the defined measurement on its own.
This work-alone capability of NIM-MCA is a strong point for applications where data must be taken simultaneously at many distant points. Each measuring spot consists of detector, NIM-crate and NIM-MCA and one needs only one Notebook/Laptop to read out the data and restart measurements. Data are then analyzed in a centralised facility.

Plug-on MCA are a recent development in particular for scintillation detectors which took the market with increasing miniaturization of electronic components. The complete MCA with preamplifier, high-voltage power supply, amplifier/shaper, ADC, MCA and USB or Ethernet interface to the PC computer are fitted into the same volume that was beforehand needed for the voltage divider of the photomultiplier alone. The complete MCA is plugged onto the 14-pin socket of the photomultiplier and it is operated from the PC. Power for the MCA is provided through USB or Power-over-Ethernet (PoE) network connection. Emulation software in the PC configures the MCA, makes live display of current data and stores spectra after measurement. The emulation software can also link seamlessly into spectrum analysis software.

The plug-on MCA makes the scintillation spectrometer completely portable and independent from mains power. The whole system can be operated from the Notebook battery for up to 5 hours of measurement and data analysis.

MCA for scintillation detectors

Light flashes from scintillation detectors like e.g. NaI(Tl), BGO, CsI, SrI₂, LaBr₃(Ce), CeBr₃ and others are normally registered in the photo-cathode of a photomultiplier tube (PMT) and the photo-electrons are multiplied over a cascade of dynodes. For operation of the dynode cascade the PMT needs a high voltage supply of typically 500 to 1000 Volt between cathode and anode. The voltage is sub-divided by a resistor chain for the various dynodes; in most applications the high voltage has positive polarity, the photocathode is at ground level and the anode at full HV. Spectrometry of negative anode signals or positive signals from the last dynode requires the usual electronic components such as preamplifier, linear amplifier, ADC and MCA for spectrum collection. Most modern MCA for scintillator detectors are plug-on units that connect directly to the PMT.

 

NIM-technology: In NIM (Nuclear Instrumentation Module) technology the standardised electronic components are operating in a NIM crate which provides all necessary voltages and where NIM modules can be inserted into standardised slots. The image (left) shows a complete NIM spectrometer for scintillation spectrometry. The small NIM-crate having six slots holds the high-voltage unit and a linear amplifier. The high-voltage feeds the voltage divider of the PMT and the measured signal is connected to the linear amplifier. The unipolar output of the amplifier goes to the input of a digital MCA model MCA8000D (the little black box). The MCA is powered through the USB connection to the Notebook and it digitizes input signals from the amplifier into 256, 512, 1024, 2048, 4096 or 8192 channel spectra. 1024 channel spectra are normally sufficient for scintillation spectrometry.
In this example the MCA uses the DppMCA emulation program for set-up, measurement control, spectrum display during measurement and spectrum storage when data taking is complete.

 

 

Plug-on MCA: The picture (right) shows a scintillation spectrometer consisting of a 3“x3“ NaI(Tl) detector and a plug-on MCA type bMCA which is connected via USB to the Notebook. The bMCA emulation software controls hardware settings, provides live spectrum display and stores data after the measurement.
The spectromer employing a plug-on MCA is independent of mains power as it can measure and analyse for over five hours from a full Notebook battery. The bMCA program is linked to the SODIGAM software for seamless transition to a high-precision spectrum analysis program which can be activated in parallel to a running measurement without interruption of data taking

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The image (left) shows another digital plug-on MCA which provides a number of additional features. For example, it has USB and Ethernet and RS232 ports for connection to the PC and one can provide external power to the unit. The DppMCA software allows to configure several input and output signals for special functions. The latter feature allows one to control experiments or measuring conditions very thoroughly

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Analog plug-on MCA: The last-but-one generation of plug-on MCA were analog units which had very similar specifications as modern digital systems in terms of form factor and power consumption. There are still many analog systems in use but 1:1 replacement is more or less impossible as these units are no longer manufactured and repair is essentially not possible.
The picture (right) shows a complete scintillation spectrometer with an analog scintiSPEC MCA which can be well used for very reliable measurement of spectra with counting rates up to 35000 counts per second.
In the case of failure of an analog plug-on MCA the unit can be exchanged without major adaptation with a digital unit and measurement can resume. The other emulation software will come with the new unit at no extra price.

Ask us for details of presented example units – we will support you as best we can.

MCA for HPGe spectrometry

High-resolution spectrometry with cooled HPGe detectors requires a number of electronic units that have well defined specifications which must be met within narrow specification limits for extended amounts of time.

Components of an analog HPGe system are:

• A high voltage power supply with switchable polarity which provides high stability of the output voltage up to + or – 5000 Volt and a current up to 10 µA.
• An analog spectroscopy amplifier with amplification in the range from *5 to *100 (minimum), selectable shaping times from 1 µs to 6 µs (minimum) and special circuitry such as Pole-Zero compensation and Pile-Up Rejection.
• An Analog-to-Digital Converter (ADC) that converts the height of the amplifier output signal into a digital number and forwards it to the MCA. There are various technical solutions how a signal height is converted into a digital number; therefore one finds various types of ADC having different properties.
  If you need more detailed information, ask us right away – we will support you as best we can.
• A Multi Channel Analyzer (MCA) that reads the digital number and stores the event in the spectrum. Most MCA also manage measuring time and deadtime and they cater to the emulation software for display of the live spectrum and other details.
• An MCA basic unit, which is a dedicated personal computer, in which spectral data are displayed, stored and analyzed. Configuration and control of the MCA normally come from this basic unit.

A digital MCA for HPGe spectrometry must provide the same functions, it only uses a special shaping network instead of a spectroscopy amplifier and a very fast flash-ADC instead of other slower units.

In many institutions one finds a large inventory of old but functional NIM modules, such as HV units, spectroscopy amplifiers and even Wilkinson or fixed-deadtime ADC. It is highly recommended to make use of these functional units because they normally are most stable and reliable. When no ADC is available as NIM module one can easily use a MCA8000D (see below) unit and connect via USB into a modern PC for the measurement.
There was a popular generation of PC plug-in MCA for HPGe spectrometry. This product line has almost completely disappeared from the market as producers of this nuclear niche product could not keep pace with changing development of PC hardware.The biggest problem were steadily increased bus data rates and the frequent changes in bus structure.
Modern HPGe spectrometers are in most cases:

• „Stand-alone“ units where all relevant functions are accomodated in one housing. Most often these units can be connected to a MCA basic unit with big screen. There are analog and digital stand-alone MCA on the market.
• NIM-technology which often have several independent MCA in one unit. Each MCA either has its own ADC, or several channels are routed over one ADC. The latter structure should only be used for low countrate applications such as alpha particle spectrometry.
• Hybrid-systems where some functions (e.g. high voltage und spectroscopy amplifier) are built with NIM-technology followed by a digital ADC and MCA.

As the number of different gamma-ray spectrometers on the market is large and there are many variants from each of them we will in the following present only one unit as an example of a high-end spectrometer.

„Stand-alone“ HPGe spectrometer:
The system is connected via USB with the MCA basic unit (Notebook or Desktop) and it uses the basic unit for data display, hardware setting and spectrum storage. The high-precision spectrum analysis program is seamlessly connected to the emulator program for very easy analysis during and after measurement. The small black HPGe spectrometer box contains high voltage up to + or – 5000 Volt, a digital shaper (amplifier), the flash-ADC, an 8k spectrum memory, counting time and deadtime control and the interface to connect with the basic unit. Connection can be made via USB or Ethernet or serial port. There is a number of I/O lines available in this MCA which can be configured to show or control various functions. The integrated software oscilloscope allows one to inspect internal signals for optimisation of data throughput and other functions. The unit has its own power supply, so the connection to the PC can be cut once the spectrometer is set-up and measurement is started.

There are too many different HPGe spectrometers and their variants on the market to mention or even show. Consult us for best advice when you plan your spectrometer – we will support you as best we can. We love to consult and find the best suited and least expensive solution.

NIM-MCA coupling

In many application one still uses existing NIM modules for amplification, handling and registration of detector signals. There is no reason to switch from NIM-technology to more modern systems, as long as NIM units are fully functional. There is also no formal breach of legislation involved as the last revision of Nuclear Instrumentation Module (NIM) Standards „Standard NIM Instrumentation System“, DOE/ER-0457T dates from May 1990 and it is still valid.

In some applications NIM-technology includes the ADC and even an MCA. In many applications, however, the HV and spectroscopy amplifier exist in NIM-technology but there is no appropriate ADC and MCA. In these case the NIM-MCA coupling through a small digital USB MCA is a very useful solution.
This configuration consists of traditional and very stable NIM analog electronic coupled with fast digital pulse processing electronics. We have very positive experience with such systems since over a decade.

The picture (left) shows a typical example of NIM-MCA coupling taking an α-spectrometer as the example. The α-chamber in double width NIM form factor contains the vacuum chamber, the detector with BIAS power supply, the preamplifier, the linear amplifier and manual vacuum control. The output signal from the linear amplifier is connected with the digital MCA8000D where a digital shaper prepares the signal for the flash-ADC. The spectrum is accumulated in the MCA and displayed and stored in the Notebook basic unit.

The spectrum length (number of channels) of the small MCA8000D is up to 8192 channels, thus this small unit is well suitable for scintillator spectrometry, alpha particle spectrometry with semiconductor and gridded-ionisation chamber detectors and gamma-ray spectrometry.
The MCA8000D is connected via USB with the Notebook; this connection serves for power supply and data exchange. The small pump next to the NIM-crate is used to evacuate the α-measuring chamber.

Whenever you have questions or need an idea for your measuring set-up please ask. We like to support you well.