Product Review: Cremat.com preamps!

[Carl Willis]

Where can you get a nice charge-sensitive nuclear preamplifier? (If you plan on doing serious work with proportional tube or plastic scintillator neutron detectors, you will need to have a preamp.)

Two options are to pay $150 or more on eBay for a used Canberra or Ortec unit which, chances are, has no documentation available and probably a front-end FET (often with no markings and hand-picked for good characteristics) toasted on arrival; or if your name is Richard Hester, you can homebrew your own design. However, there's an alternative to the "homebrew challenge" or the "eBay gamble" that I'm sure will be attractive to many in our community: the $55 Cremat CR-110, available with full documentation at www.cremat.com. For another $55 if you choose, you get a board to mount it on that includes power supply regulators, bias voltage RC filtration, and a 200M detector load resistance. I'm happy to report that this small package works great with proportional tubes--see the BF3 neutron spectrum below and compare with my spectrum taken a few days ago with an Ortec preamp.

The CR-110 is a module that plugs into a SIP socket and is about the size of a quarter. Apart from the low cost, it has some notable advantages over other preamps. Perhaps the most practically relevant to us might be the small size and flexibility: packaging is up to you. Also, the noise figure and rise-time quoted for the CR-110 compare favorably with Canberra or Ortec preamps. It consumes very little power and requires a simple supply: +/- 6-13 VDC. I see no reason to tie this preamp to a NIM bin when it can be driven days on end with a couple local 9 V batteries.

There are some disadvantages worth mentioning. The circuit has no protective diodes on the input. Certainly this helps the CR-110 beat the noise figure for the protected Canberra 2006, for example, but if static pops that sucker you are out $55 instead of just one FET. Plan to add some diodes to clamp the input, or just be extra careful (use a continuously-variable HV supply, make sure connectors are clean and won't spark over, put a terminator on the input when not in use, etc.). Other possible disadvantages are the small linear output range (+/- 3 V) and the rather long feedback RC time constant (140 microsec.): for high count-rate applications, pileup-induced saturation could be more a problem with the CR-110 than the common big-name preamps (which can do +/- 10 V and have shorter time constants and perhaps fancy active reset features). Finally, although buying a CR-110 saves you from the tricky aspects of design and fabrication, you still have to apply some sense when making your practical installation with it. The input leads must be rigid and have high-quality insulation and low capacitance; ground loops and sources of microphonics must be avoided.

My preamp, shown below, is housed in a die-cast aluminum box. It comprises both the CR-110 and the CR-150-AC. Two 9V batteries supply power through a switch. The preamp output BNC socket is of the "floating" type, with the barrel tied to the circuit board ground. The detector leads from the board to the input BNC are bare #10 wire.

So here are my final recommendations. The CR-110 at $55 is a great deal. Though it works great as part of a NIM-based detector system, I figure it's niche is really in applications where you want to have a small self-contained detector, or want something free from the constraints of a NIM power supply. The "test board," CR-150-AC, is not something every user will want or should get. The supplied load resistor and HV filter resistor are really meant for proportional tubes and are much too high if using a scintillation detector. I believe that most users will want to customize on the support board, could make something even better than the "test board," and won't want to pay $55 for it. For plug 'n' play convenience it's nice.

-Carl

[Jon Rosenstiel]

Carl,
You must have got their attention... I see they are planning (spring of 2006) to release the CR-111, which is basically the CR-110 but with faster rise time. Also due for release is the CR-112, which is tailored for PMT use.

http://www.cremat.com/CR-111.htm

Jon Rosenstiel

[Steven Sesselmann]

Carl and Jon,

Is the Cremat product still the product of choice for an He3 tube preamp?

If so, which type would be my best choice?

Steven

[Jon Rosenstiel]

Steven,

Can't offer anythng on the Cremat cause I haven't fooled with any of em'. I've stuck with the traditional charge sensitive preamps. (Ortec 109PC and / or Tennelec TC-175).

Jon R.

[Carl Willis]

Hi Steven,

The two components I discussed in this review are all I have used, and they are still serving me fine. Still a bargain. I hope the company continues to make them.

-Carl

[Steven Sesselmann]

Carl,

I am still learning about neutron detection methods, so please be patient with my questions..

Can I use a preamp between the detector tube and any survey meter, or bench meter, or are the preamps only used in front of some Nim modules?

Steven

[Carl Willis]

Hi Steven,

The Cremat preamp could be used in front of almost all ratemeters, even the simplest Monitor 4 or CDV-700. These instruments cannot be used to provide bias to the tube (voltage is too low), but they can be used as readouts for a preamplified signal if the signal is AC-coupled to the detector input.

For use with multichannel analyzers, or in other situations where the signal will be analyzed for energy or timing information, it is customary to follow the preamp with a Gaussian shaping amplifier. These are a very common and essential NIM component. Canberra, Ortec, Tennelec, and just about everyone else make several flavors of shaping main amplifier.

Best regards,
Carl

[Steven Sesselmann]

Thanks Carl,

I will try to get one of these Cremat preamps to play with ..

Steven

[Steven Sesselmann]

Carl,

I received my Cremat-110 today, and putting it together seems quite straight forward, but I am a bit confused about the bias voltage port.

If I were to use my He3 tube with say a Ludlum rate meter, then the rate mater would supply the bias of 1687 V to the tube, but then where would you connect the output from the preamp?

There seems to be two connectors where I was expecting one.

Steven

[Carl Willis]

Hi Steven,

In the situation you describe--wanting to run this preamp in a "one wire" configuration with a ratemeter that is also the souce of HV--you have to AC couple the pulse output of the preamp onto the cable from your ratemeter, and you have to isolate the preamp input from the cable.

So you'll put a capacitor, probably in the ballpark of 0.01 uF, between the preamp output and your cable connector, to block the HVDC on the cable and couple the preamp signal onto it.

Then you need to keep the output pulse you coupled onto the cable from getting fed back into the preamp input through the counter tube load resistor. Use either an LC or RC low-pass filter upstream of the load resistor to decouple the preamp input from the cable.

-Carl