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MattiaB
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Hello everybody

Post by MattiaB » 26 Oct 2016, 09:52

Hello folks, I'm Mattia Beninati from Italy,
i'm always been interested in radiations and i started gamma spectroscopy since Agoust with a Theremino pmt adapter and a Ludlum 44-3 LEG probe from my Ludlum 3 ratemeter, wich introduced me in radiation measurement 2 years ago.
After a few trials i get to the conclusion that has to be some kind of problem with the crystal as i couldn't get nice spectrums from it.
I then bought from the bay a used Scionix 1.5x2.25 Nal(TI) detector and after reading literally all the post on this forum i finally get it back to work building a new voltage divider for the pmt after some trial and error with the resistors.
I think i found a good compromise between resolution and linearity and i will be glad to share my results with you.
Actually i obtain my spectrums with Theremino MCA but i'm working on gettin good result with Becqmoni as many of you from the forum suggest.
Attached are some spectrums i made recently.
Thanks everybody for the help and the informations i found on the forum.
Mattia
Attachments
th232.jpg
th232.jpg (460.61 KiB) Viewed 3464 times
cs137 scionix with compensation.jpg
cs137 scionix with compensation.jpg (459.71 KiB) Viewed 3464 times
am241.jpg
am241.jpg (437.45 KiB) Viewed 3464 times
Mattia Beninati
Venice, Italy

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Steven Sesselmann
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Re: Hello everybody

Post by Steven Sesselmann » 26 Oct 2016, 11:56

Ciao Mattia,

Welcome to the forum.

Yes you should try PRA and Becqmoni software, they works quite defferent to Theremino and in my opinion more accurate, the convolution function in Theremino makes the resolution look better than what it actually is, but nothing is actually gained in the convolution process. It is like stretching a rubber band, it becomes longer but you still have the same amount of rubber ;)

Steven
Steven Sesselmann | Sydney | Australia | gammaspectacular.com | groundpotential.org | beejewel.com.au |

luuk
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Re: Hello everybody

Post by luuk » 27 Oct 2016, 02:28

Hello Mattia,

Looking at your Cs-137 spectrum and then resolution from 6.1% you ot there, I can only say that is something incorrect.
The 662 KeV peak has a feet(slope) on the left side, so with that a resolution from 6.1% is none realistic, if the peak was perfectly formed than it could be 6.1% but not how it looks now.
I think there should be some adjustments in your mca stetting to get is right and give a real realistic resolution.

Regards,
Luuk,

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Steven Sesselmann
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Re: Hello everybody

Post by Steven Sesselmann » 27 Oct 2016, 12:37

Luuk,

You have brought up something that has puzzled me for a long time.

Whenever I have a crystal with bad resolution it always has that typical left shoulder.

Do you know what causes it and if it can be fixed?

Steven
Steven Sesselmann | Sydney | Australia | gammaspectacular.com | groundpotential.org | beejewel.com.au |

MattiaB
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Re: Hello everybody

Post by MattiaB » 27 Oct 2016, 12:53

Hi Luuk,
thank you for your reply, the cs137 spectrum i posted was made with "resolution compensation" on theremino.
I just made a new onen without compensation.
Does it look better?
The fwmh is now +2%, a big difference, do you think i should work more on the voltage divider?
Now i use 20M between K and D1, 10M for the dynodes and 20M from D10 to A.
The detector is a 38B57 1.5M-E1 sn.SED875

Regards
Mattia
Attachments
cs137.jpg
cs137.jpg (456.25 KiB) Viewed 3444 times
Mattia Beninati
Venice, Italy

luuk
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Re: Hello everybody

Post by luuk » 27 Oct 2016, 15:47

Hi Steve,
The left shoulder is caused when a crystal is a little(partly) wet, the light produced in the wet part will give a lower light output than the dry crystal therefore the peak is shifting to the left and gives a shoulder or a wider peak.
Luuk

luuk
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Re: Hello everybody

Post by luuk » 27 Oct 2016, 15:51

Hello Mattia,
Yes this looks like a good and realistic spectrum and resolution.
No chancing the voltage divider won’t give you a better resolution, it is just the crystal.
Luuk

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Steven Sesselmann
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Re: Hello everybody

Post by Steven Sesselmann » 27 Oct 2016, 17:16

Luuk,

Thanks for enlightening us about the wet crystals, I quite often see this shoulder and thought it had something to do with the PMT, but now I know.

Steven
Steven Sesselmann | Sydney | Australia | gammaspectacular.com | groundpotential.org | beejewel.com.au |

MattiaB
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Re: Hello everybody

Post by MattiaB » 28 Oct 2016, 05:47

Ah ok.. :(
Luuk, This let me think about the operative life of a crystal, for how long they work good?
In my specific case, as my detector is used it's an issue of time or how the probe was used?
Thank you
Mattia
Mattia Beninati
Venice, Italy

luuk
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Re: Hello everybody

Post by luuk » 28 Oct 2016, 08:29

Hi Mattia,
The life time of a detector depends on a few things.
Normally the crystal can live almost for ever as long as it stays dry, only one exception high radiation dose can make defects in the crystal structure and kill the (dry)crystal.
The pmt can work under normal conditions for about maybe 15-25 years, it will get less sensitive at the end of it’s lifetime, also depends if the detector was used with a relative high dose than the pmt will lose it sensitivity more quick and will need more and more high voltage to keep on working until it is not useful anymore.
Main problem with detectors is that the seal breaks after some years and the crystal is slowly exposed to air(moister) and will get wet, only solution is to take it apart dry and clean it and reassemble it again, a wet crystal can be cleaned without any problem for many times to extend it's liftime, unless it is exposed for a to long time to humidity than it will slowly melt away and is completely useless and repair is not possible.
Luuk

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