Hi guys,
At Scionix we recently made a large Compton suppression or antiCompton system, and because the main NaI(Tl) detector is made out of
one very large piece of crystal I thought it would be interesting for you to
see how it is made.
FYI: An Anti-Compton/Compton-Suppression system is used together
with a HpGe detector (and sometimes lots of plastic scintillators all
around) and takes care of the Compton suppression so the background
made by the Compton effect is suppressed a few hundred times (can be
up to >800 x) depending on the total configuration. This is done by the
use of coincidence or anti coincidence electronics, and therefore it is
possible to see very small peaks that normally are lost in the
“background”.
These systems are mainly/only used in high energy physics!
We start with a large NaI(Tl) crystal that is ±38cm(15”) long and ±23cm
(9”) round, the total weight is around 50Kg(>110lb) so that is a ”big boy”!
To start, there has to be made a large hole >100mm (>4”) in the crystal
over its total length, this will be done on a big lathe.
And that is not as easy as it looks the crystal besides that it is, very
expensive is also fragile, sensitive for temperature variations or
mechanical chocks, so a small mistake can make it crack or even break
completely, too much speed or not enough cooling wrong type of cutting
tool etc. etc. and the crystal can end up as a large pile of very expensive
scrap.
We have one large crystal expert, that is my colleague and friend Tom
he knows all about the best way how to machining, and also knows all
about treatment and compensation from these extreme large the crystal
he does this work for a long time and is really good at it!
I can honestly say that he is probably the only one (maybe even
worldwide) that can do this specialistic work with high precision use of all
knowledge he got and lots of patience. No one ever made a detector like
this before!
The crystal is difficult to handle because of the large weight, so it takes
two man to get it fixed in the jaws of the lathe.
The lathe is standing in our semi dry-room that is the only place to work
on the crystal with a (metal)lathe without getting the lathe it in to a big
block of rust within a few hours.
Note: NaI(Tl) is an aggressive material it starts to oxidize every piece of
metal that comes in contact with it when there is too much moister near
the crystal.
The crystal has to be stabilized and balanced before the lathe can be
safely turned on.
Once the crystal is fixed and stable in the lathe, the real work can start
The first contact between the lathe’s cutting tool and the crystal is a
critical moment, the crystal could crack if it is not done with great care.
There is a lot of plastic wrapped around the lathe, because the oil will
splash all around, and a slippery floor is not what you want.
The material is cut out, but only a small part at the time it is a slow
process so in total this can takes a few days for such a large crystal
before it is completed.
Because the crystal is so large it is impossible to make the hole from
one side in one go, therefore the crystal has to be turned halve way the
process.
The turning around of the
crystal is a tricky moment and
again has to be done very
carefully a small mistake and
the crystal can break.
Now the hole is made from the other side and if all is good, they will
meet in almost the middle of the crystal.
Finally, as you can see the hole is through it looks like that all went as
planned and no problems like cracks at all.
Now it needs the finishing touch, to make it exactly the right size and
completely smooth.
When the crystal is ready again two man are needed to take it out of the
jaws of the lathe, the crystal is now lighter but weights >30Kg (>66Lb)
and is still difficult to handle.
The crystal lost about 19 Kg material because of the large hole.
This is Tom our large detector expert!
Crystal ready to be treated.
The crystal has to be treated and compensated that is again a timeconsuming process that is done by hand and takes a lot of experience
and pre-testing that also can takes a few days.
The target is to get the best resolution measured with Cs-137 in the
center of the well, but it has to be homogenous everywhere in the well.
During this process the crystal is several times pre-tested to see if the
treatment is going in the right way, so for the best resolution and that the
crystal will give a homogenous response.
If results are not as expected then there has to be made a decision what
will be the best compensation method and were it has to be
compensated to get the best result, this is a joint effort between the
expert and the tester, sometimes this can take a long time before the
results are as expected, but in this case it gave no real problems.
The detector is pre-tested with 6x 2” pmt’s, I preselected the pmt’s and
matched them so that they all give exactly the same gain, once the best
resolution is achieved the crystal can be mounted in the low background
stainless-steel housing.
The Crystal in the stainless steel housing!
Next thing is to glue the glass light guides on the crystal this is done
because the crystal will be sealed around these light guides.
After this is completed again a pre-test is done just to check if all is still
working as before and nothing went wrong while mounting the crystal in
the housing, then the top plate can be mounted.
After the detector is completely sealed, things are getting even more
interesting because this large detector will be readout not with standard
pmt’s as in the pre-test, but with a large amount of SiPm’s arrays, the
main reason is that they are not sensitive for large magnetic fields in
were this detector is used.
This is the first large anti Compton system that is readout with SiPm’s no
one did this ever before!
Each pmt’s is replaced with 4 arrays from 2x2 SiPm’s so for the
complete detector 96 SiPm’s from 6x6mm.
All SiPm’s are couplet parallel and a temperature compensated
amplifier will process all the signals in to one output.
The result is a large but low-profile anti Compton system and no pmt’s,
mu-shields or high voltage needed, it and can run on only +5V.
The system is now almost but not yet complete, it needs two so called
plugin detectors one detector will close the main detector at the end if
needed, the other will be placed around the “neck” of the HpGe detector
once the anti-Compton system is in use it will completely close the
detector.
When all detectors are mounted together in this system will give a 4𝜋
coverage.
The two plugin detectors also NaI(Tl) will have a low background
stainless steel housing as well.
This is one of the plugin detectors it has 6 SiPm arrays from 2x2.
The other plug-in detector is in two parts so that can be fitted around is
the “neck” of the HpGe detector and has also a total of 6 SiPm 2x2
arrays.
Both plugin detectors mounted in the main detector.
This complete detector with the plugins weights because of the heavy
stainless steel housing > 50Kg(>110 lb).
I think it turned out to be a great and nice-looking detector and the
working of it is really good.
Resolution of the main detector with 6 pmt’s is 7,1% that is extreme
good for such a large detector.
But with the SiPm’s the resolution is degreased to 9.4% that was to be
expected because the coverage of the surface with the SiPm’s is only
about 35% compared to the pmt’s so that is the best possible resolution
with this configuration.
Luckily for the application were this detector is used in, the resolution is
not the most important part that is the Compton suppression and the fact
that it can work in a high magnetic field.
As always I do hope that you found it interesting to read this report, any
questions feel free to ask.
Luuk
20-04-2020
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