FAQ - MCA , Just th' facts.

[Richard Hull]

What is an MCA?

MCA stands for Multi Channel Analyzer. PHA is the mode that we tend to use the device in, however. (Pulse Height Analyzer). To see an image of one in action, look at the post called "New Canberra Model 40" that I posted in October of 2003.

How does it work?

A signal consisting of, often, irregularly spaced, but very fast arriving pusles is intercepted and EACH PULSE is analyzed based on a measurment of its pulse height which is converted to a digital number/signal (ADC - analog to digital conversion). This digital number is then summed in a channel that is set aside in memory where other pulses of its exact same amplitude have been placed.

The channel values are based on the ADC conversion level setting adjusted by the user to represent some convenient ratio or function of the input signal energy. i.e. channel #1= to 1kev energy, channel #115= to 115KEV, etc. Typically, the maximum number of channels can be selected by the user. The most common channel number ranges are 1024, 2048 and 4096. The limit on this setting is usually a function of the available internal memory size.

Older MCA's back in the 50's were lucky to have 256 channels and a 512 channel machine was a real research engine! By the 70's, 1024 was stock and memory expansions could be purchased for a few hundred more dollars which would allow 2048 channels. In modern machines, the sky is the limit with no memory premiums.

Speed is important..........

A critical period of time called the conversion time is literally a dead time for the instrument where incoming pulses will be missed or ignored. This time is determined by the resolution and inherent speed of the ADC electronics. 8 bits will take longer to convert than 4 bits, etc. Lower resolution means less precise height analysis of the incoming pulses. Most ADCs associated with MCAs are in the 200mhz range. This refers to the main clock and not conversion rates. From here the resolution and internal conversion electronics determines how fast you can work and on just how "hot" a material you can sample without dropping or missing pulses.

In general, a 12 bit converter should work at a 10mhz conversion rate. If one knows that one will never be counting fast, then a greatly reduced rate can suffice. However, it is critical to remember that nuclear disentigration is random and the pusles are irregular. (i.e., An average ten hertz pusle rate can have three pulses a few tens of nanoseconds apart and still be a ten hertz signal. A general purpose MCA, however, can't rely on this slow rate being evenly spaced.

In actual use, in order to avoid problems in the results, an MCA operating in the PHA mode doesn't like to have pulses arriving for conversion any faster than 1/10th of the maximum conversion rate. This is often displayed as "dead time" on a meter on many MCAs. If the dead time rises above 10% then the norm is to move the source farther from the detector to assure that no pulses will be missed and that the meter reads 10% OR LESS.

An MCA is a complex assemblage...........

In general, a full MCA is a string of electronic devices. It will often include a preamplifier for the nuclear detector that can have its gain set to any desired level. Shaping circuits to condition the input signal pulses to a standard format that the ADC can handle. This is followed by an ADC which drops the converted pulse height value into the proper channel bins once digitized.

From here on, the signal is pretty much digital and memory cells of the full resolution width are constantly updated after each conversion, adding to the total counts in the various cells as needed at the end of each conversion period.

The memory is often continuously and synchronously dumped via vertical and horizontal signals to a screen for viewing on a full blown MCA system so that the human eye and brain can absorb all the data.

This is virtually always displays on the horizontal graph axis as bin or channel numbers which represent some function of the energy of the detected particles. The vertical axis of the graph represents counts or the number of particles detected for each energy bin or channel. The result is an easy to read display of an energy spectrum.

Energy Spectrum of what?

All the above assumes you have a detector or transducer that is capable of producing pulse heights that are a function of the energy of the particle being detected. Such detectors are quite special and not all detectors are capable of this.

The most common detector used with the MCA is a Sodium Iodide detector, (NaI:Th), coupled to a photomultiplier tube, (PMT). This detector is sensitive to Gamma ray photons only, for the most part. (see FAQ on the PMT).

This is the detector that most of the fusor people doing neutron activation equip themselves with.

The versatile MCA......

Some MCAs have several modes beyond PHA. Some can do TPA (timed pulse analysis) or MCS (multichannel scalers). This mode actually has several names, but its claim to fame is that you may set an interval and pusles into the unit are simply counted over this interval and at the end of the interval the total count is placed in channel #1. This cycle mindlessly repeats over as many intervals as there are channels. The result is a graph of the time ordered count. This is great for short lived isotopes to determine half life via exponetial decay. Just set it and forget it. Come back later... see your decay curve and do the math.

There are also sum and subtract modes. You can subtract background counts and continue to collect and often much much more. So the instrument can do a lot of things where time and voltage need to be ordered and organized to get useful data.


Who needs an MCA?

Only those people needing to identify radio isotopes or gamma ray energy signatures will find the device useful. The MCA can also be considered a very expensive, doubley indirect, neutron detector (not counter) for fusors that are fusing very well.

As an isotope detector/identifier, the MCA is of great value in probing the results of neutron activation studies. This is a very oblique way of detecting when a lot of neutrons have been absorbed by a piece of metal or other material object.

The MCA will not identify the energy of any neutron beam or stream. It will not directly detect neutrons. For the fusioneer, the MCA is a device that might be considered once healthy and or heavy fusion is being done. It should certainly only FOLLOW the acquisition of a neutron counter and never preceed it unless it falls into one's lap.

Can I roll my own MCA?

Sure, if you have the requisite skill sets. Modern electronics is great and computers are now the norm as MCAs have become uncoupled from the old massive all in one instruments of the 1940's thru the 1980's. Now, a person can just design a preamp and signal conditioning electronics and feed the signal to an ADC which is linked to a properly programed computer port. The rest is just working to time the software to operate the system, ADC and display the results. Such work is for a full fledged computer person with some electronics skills and is not for the casual experimenter.

Snags and caveats......

Most MCAs are found on E-bay and are either not working or if working never have an associated detector sold with them. Thus, you will have to purchase a users manual/service manual. You will have to ferret out and hookup a working PMT and Crystal. You will need a separate, adjustable (0-1000 volt) high voltage supply for most MCA setups to power up the detector head. You will have to also puchase at least one reference source of radioactive material to calibrate the device.

Many manufacturers have long ago abandoned the older MCAs they have made and might not be of much help if you have a very old unit that needs a manual or servicing.

Modern MCA's, often no bigger than a pack of cigarettes with software can be had NEW for under $3,000. As the money has inflated over the years, I will offer a comparison based on equivalences over the years. In the 1950's A working MCA with display, detector head and 512 channels was about the price of 5 brand new, showroom floor, full sized automobiles. By the 1960's a 1024 channel machine was about the price of 3 new automobiles. In the 70's and 80's it was about 1.5 new automobiles. In the 90's about 1 automobile for a 2048 machine. Today, if you have a computer, it is about 1/6 of a new automobile, and this is for unlimited channels.

E-bay items of unknown value or functionality can go for $200.00

In closing, be sure you need one of these items and have the guts, money and nerve to see it through the rather complicated path to owning one that is functional and accurate.

Many thanks to Dave Trimmell for including a lot of factory URLs for those wishing to really immerse themselves in the nearly infinite amount of details that they have thoughtfully placed online.

I was going to do this at the end of this FAQ post as I built it, but Dave has probably produced a nicer and more concise list than I would have provided (see his posted reply for these URLs)


Richard Hull

[davidtrimmell]

Some sources that may be useful to Richards MCA FAQ:

http://www.canberra.com/literature/933.asp
http://www.canberra.com/literature/935.asp
Other interesting literature on Canberra's website also.

http://www.ortec-online.com/application-notes/an59.pdf
And more interesting stuff from Ortec:
http://www.ortec-online.com/application ... -notes.htm

BTW, Richard I love your FAQs, keep em coming.

David Trimmell

[Starfire]

I second David - I love'm too - I would like to see a FAQ forum. This is core information and your efforts are much respected and appreciated Richard.

[lambda]

I agree with the others...
these FAQs go straight into my FAQ binder. I would like to see these along with all the most informative posts in their own section.

Trevor McWilliams

[Richard Hull]

Not to be contrary, but all FAQs related to a specific subject forum are all in the proper forums now. Just search on "FAQ" and nothing else and all FAQs in that forum WILL be pulled up ready and organized for you along with all related comments. The related reply comments often add to the knowledge imparted.

If you just want my FAQs and no reply comments by others then further narrow the search to "FAQ" and search on author name "Richard Hull".

Not a real tough deal here folks. Our search engine may not be the best but it is pretty darned good. This is why ALL of my FAQs start with "FAQ (spc) - (spc)" then subject.

Finally, the FAQ is not owned by me. Others are welcome to chime in if they feel like authoring a FAQ. All that I ask is that for the sake of consistency that you start with "FAQ" as noted above. Certainly, FAQs should be limited to dealing with a technical issue that is of broad and continuing interest.

Richard Hull