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MILAND

Possibilities for the readout of gas scintillation detectors for neutron scattering

applications.

MILAND

To meet the MILAND specification requires the ability to build a detector modulewith dimensions 320 mm x 320 mm and with 1 mm position resolution. A neutrondetection efficiency of > 50 % at 1.8 is also required. This latter entails a 3He

pressure of 5 - 10 bar and an active detector thickness of 30 mm. To obtain therequired position resolution a suitable stopping gas is required, e.g. ~ 4 bar of CF4.Position errors due to parallax are also an issue and need to be taken into account.Count rate specification is given as a local counting rate of 100 kHz / cm2 and a globalcounting rate of 1 MHz. By the end of the first 18 months of the project the intention

is to have built various types of 128 x 128 channel prototype detectors for evaluationpurposes.

Gas Scintillation Detector

Francesco is proposing a 3He / CF4 gas scintillation detector, GSD with a GEM foil toprovide a very high light output neutron sensitive scintillation detector. This systemhas two emission bands, one at 300 nm and another at 600 700 nm. As such it is

possible to detect the emitted light using both bialkali PMTs and CCDs.

Question: Do you need to use quartz windowed PMTs? Is there any significantemission in the UV region?

Upon neutron absorption in 3He a proton and a triton are produced. These will causelocal ionisation within the gas volume. As the ions decay to their ground states

primary scintillation light is produced. The time constant for the primary lightemission is governed by the lifetime of the triton and proton. The electrons drifttoward the GEM plate. In the vicinity of the GEM plate secondary ionisation occurs,which results in the emission of secondary scintillation light. This will have a timeconstant of a few 100 ns.

Advantages of GSDs

In pursing appropriate lines of development of the gas scintillation detector forMILAND, there must be a realistic opportunity of achieving the MILAND objectivesand the opportunities must be at least as good as those for equivalent detectors usingeither MSGD or MWPC or a solid scintillator.

GSDs have a number of advantages. The light output is very high with up to 108

photons produced for each detected neutron. This is 2 3 orders of magnitude greater

than for ZnS/6

Li, the brightest solid state neutron scintillator currently in use. Inaddition, whereas ZnS/6Li is opaque which imposes a large dynamic range in the size

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of the output signal, there is no corresponding large dynamic range associated with agas scintillation detector. There may well be some speed advantages compared withsome designs of MSGD where for example the capacitance of the MSGD plate slowsdown signals taken from the rear side of the plate.

However, understanding the gas gain obtainable from a GSD operated with a GEMrequires further research. A gain of 108 is only available if the GEMs can operate at ahigh gas gain. The DELFT group have found that the gain of GEMs are limited whenoperated in high pressure CF4. It is not clear whether this limitation is in somewayspecific to the DELFT setup or whether it is fundamental to the presence of high

pressure CF4. If the latter is the case then at a partial pressure of 4 bar there may beno gas gain at all from the GEMs and the magnitude of the light output will be limitedto that produced by primary scintillation.

Even if the light output of the GSD is significantly reduced by the CF 4 content, thelower dynamic range of the GSD signals compared with ZnS/6Li scintillator may still

result in a detector with a superior performance.

Question: What is the number of photons produced by primary emission and overwhat time period are they emitted?

Question: How many GEMs do you expect to operate with the GSD?

Question: To date what is known about the photon production per electron and perneutron in 3He / CF4 mixtures as a function of gas mixture and pressure?

Question: Under what conditions can you get a neutron / photon ratio of 107 - 108?

Alternatively, this gas gain limitation caused by the presence of high pressure CF4may be overcome if a MSGD is used instead of a GEM, where sufficient gas gain can

be obtained despite the high partial pressure of CF4. However, unlike the GEM,scintillation light will not escape through an MSGD fabricated from Schott S 8900glass. An alternative arrangement might be to reorient the detector so that neutrons

pass thorough the MSGD before they enter the drift volume. The MSGD could beviewed through a window on the other side of the detector, but a transparent driftelectrode will be required.

Question: Do you have an anode between the GEM and the CCD / PMT readoutdevice and therefore do you already have a transparent electrode?

A question is whether such a device would still have any advantages over aconventional MSGD. Since the MSGD in the GSD is present only to provide gasgain, it does not have to be a 2D device.

Integrating readout options

Integrating devices such as CCDs provide one option for reading out GSDs and allow

track reconstruction which can be used to determine the centroid of the alpha andtriton production and thus yield enhanced position resolution.

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However, for ISIS and other pulsed sources, individual event counting is required inorder to determine neutron energy by time of flight analysis. Current CCDtechnology has readout times which at best are ~ 1 ms. These are too slow forneutron time of light methods at pulsed sources where typically timing resolutions of

1 10 s are required.

Photon counting readout options

Several methods are available for individual event readout. These include:

1. LIPs PMT readout2. Anger camera readout3. Position sensitive PMT readout4. Wavelength shifting fibre readout

5. ISIS fibre coded readout

1. LIPs PMT readout

In this system a 60 mm x 60 mm detector area 30 mm deep could be read out byviewing with four 30 mm diameter PMTs arranged on a square array. By using pulseheight measurements and timing measurements, it may be possible to determine notonly the geometric centre of the image of the charge cloud, but also determine thecentroid of the event and the depth of interaction within the gas volume.

Question: Is it possible to detect the primary light? If this is the case does the timedifference between the primary and secondary light provide information on the depthof interaction in the gas volume?

All four PMTs have to be sufficiently far away from the detector to view all of the 60mm x 60 mm x 30 mm volume. The geometric centre of the image of the charge cloudcomes from measuring the relative pulse heights in the four PMTs whilst centroidweighting can come from measuring the time of arrival of the signal in the PMTs as afunction of position.

Question: Has this system been tried for the electron polarimetry case?

Question: Have you tried it for neutrons?

Question: Were your simulations for electrons or neutrons?

There is a trade off here between pumping up the partial pressure of CF4 so high thatthe centroid has little influence on the required position resolution. However, underthese conditions the gain of the GEMs may be so limited and the s/n is very poor.Alternatively, the GSD can be filled with CF4 at a lower pressure, where the GEMgain and hence s/n is higher, but then centroid calculations become important in

achieving the required position resolution.

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The LIP approach could enhance position resolution, by providing information onboth the position of the centroid of the reaction products in the gas and the depth ofinteraction in the gas. One concern is that a considerable effort will be needed in todesign the electronics to achieve very stable and reproducible PMT and electronics

performance to accurately characterise the arrival of the primary light and/or measure

the change in position resolution on a 100s ns timescale.

On the basis of a 30 mm detector providing sufficient position resolution the numberof PMTs required for a 128 mm x 128 mm detector is 36, allowing for end PMTs andthe number of PMTs for the 320 mm x 320 mm detector is 169.

2. Anger camera readout

If single anode PMTs are used, an Anger camera could be envisaged which is thesame device as that proposed above, but relies only on measuring the relative

intensities of light in the various PMTs. This provides position information of thegeometric centre of the event, but there is no obvious way of calculating the centroidof the event. Therefore a high CF4 pressure is needed to minimise the influence of thecentroid on position resolution.

Again, on the basis of a 30 mm detector providing sufficient position resolution thenumber of PMTs required for a 128 mm x 128 mm detector is 36, allowing for endPMTs and the number of PMTs for the 320 mm x 320 mm detector is 169.

Multi anode PMTs provide a interesting way of constructing an Anger camera, and 50mm x 50 mm flat panel devices from Hamamatsu are particularly interesting. Thecurrent flat panel Hamamatsu PMT has sixty-four 6mm x 6mm pixels and a newdevice which should be available this year has two hundred and fifty six 3 mm x 3mm pixels.

Using these devices will enhance the position resolution of the detector, but the costwill be the increase in the number of electronic readout channels required.

A 128 mm x 128 mm prototype detector would require nine 6 mm pixelated PMTsand 576 channels of electronics, and the 320 mm x 320 mm detector would require ~forty-nine 6 mm pixelated PMTs and 576 channels of electronics. The corresponding

numbers using the 3 mm pixelated PMT would be nine PMTs with 2304 channels ofelectronics and forty nine PMTs with 12 544 channels of electronics!

One advantage of the pixelated PMT for Anger camera technology is that only oneextra layer of pixels is required around the perimeter of the detector to readout theactive detector area. In the case of the single anode PMTs one whole PMT is neededaround the perimeter of the detector. From this point of view the pixelated detector ismore efficient and allows more efficient stacking of neighbouring detectors modules.Hamamatsu are developing ASICs to go with their 256 channel PMT and once these

become available, dealing with the relatively large number of electronic channelsbecomes easier. However, unless these ASICs can easily be interfaced with counting

electronics a significant effort is still needed to handle the realise a counting system.

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If the light output from the GSD is sufficiently high, it may be possible to utiliselarger pixels than the 6 mm pixels of the Hamamatsu flat panel detectors. One optionmight be to couple the PMTs to the GSD via tapered light guides to reduce thenumber of ADCs and electronic channels required.

3. Position sensitive PMT readout

Bruno, I dont remember all the details of this device. I think you said it was a 75 mmdiameter PMT with a single anode. The dynode arrangement must allow the positionof the event on the photocathode to be preserved on the anode. Readout is then viathe induced charge on two sets of orthogonal wires, giving 64 readout channels perPMT.

Question: Bruno, can you provide the name of the manufacturer of this type ofdevice?

Like the wavelength shifting fibre readout in section 4, the position sensitive PMTreadout can either be analysed directly to determine the centre of an event or fedthrough ADCs which then provide an opportunity for interpolation and a moreaccurate position determination.

Four PMTs will cover a 128 mm x 128 mm detector.

Since the intrinsic resolution of the PMT is ~ 8 mm, (75/8?) ADCs and interpolationis required. Each PMT therefore requires 64 ADCs and 64 channels of discriminatorelectronics. The 128 mm detector might require 256 channels of ADCs anddiscriminator electronics whilst the corresponding number for the 320 mm x 320 mmdetector could be 16 PMTs and 1024 channels of ADCs and discriminator electronics.

If the ADCs and detector electronics can utilise an existing ASIC then the investmentin readout electronics is manageable. If an ASIC in unavailable, the number ofelectronics channels presents a significant management problem in terms ofmanufacture, setup and maintenance.

Bruno already has a PMT of this type together with 64 channels of electronicsreadout. This was designed for wavelength shifting fibre readout experiments and

could be made available for initial investigations into the potential of positionsensitive PMTs for GSD applications.

Question: Bruno, do you have an interpolating decoder that is providing accurateposition information and is there a possibility of be able to feed this device into theISIS DAE with a gated input.

Division of the anodes into 8 x 8 readout channels may be more than required,depending on the efficiency of the light collection and therefore the accuracy of the

position interpolation. If this is the case it may be possible to reduce the number ofelectronics channels either by ganging together the anode readout wires or optically

coupling the photocathodes of the PMTs to a larger surface of the GSD window with

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some form of tapered light guide as suggested in section 2. This latter approach hasthe added advantage of also reducing the number of PMTs required.

4. Wavelength shifting fibre readout

A number of groups have begun developing wavelength shifting fibre readout systemsfor use with neutron sensitive scintillators. These fibres are readily available in 1mmor 0.5 mm diameters or cross section, and typically absorb light in the 425 nm regionand emit light in the 494 nm region.

Question: Is it easy to put additives in the gas to change the emission wavelength?This might be quicker that getting fibre manufactured with a specified absorptionspectrum.

Crossed arrays of wavelength shifting fibres readout systems can provide two-

dimensional position information.

At ISIS we are developing a one dimensional position sensitive detector based onwavelength shifting fibre readout and ZnS/6Li scintillator. In combination with thedetector electronics developed for the GEM diffractometer at ISIS these detectorsexhibit expected neutron detector efficiency and good insensitivity to gammaradiation.

Since the light output of the proposed GSD is expected to be significantly brighterthan ZnS/6Li it should be possible to develop a position sensitive readout systemsimilar to that being used for the solid state scintillators.

Some form of optical collimation may be useful to limit the extent of the light spreadon the wavelength shifting fibre readout arrays.

One disadvantage of the GSD is that the neutron / gamma discrimination may not beas good as that of ZnS/6Li. In the latter case the high alpha / beta ratio facilitatesneutron gamma discrimination. Is the alpha beta ration of the proposed GSD likely to

be as good as ZnS/6Li? This may be offset by the higher light output of the GSD.

Question: What is known about the alpha / beta ratio for the proposed GSDs?

Some electronics development will be required to find the centre of the light spreadon the wavelength shifting fibre arrays. This could be done digitally, giving a

position resolution in the order of the fibre diameter, or by interpolation, giving aposition resolution less that the fibre diameter. At ISIS we are only developing thedigital version and have no interpolating electronics.

Use of an X / Y readout system requires 640 readout channels for the full sizedMILAND module, whilst the 128 channel demonstrator would require 256 channels.

It may be possible to code the wavelength shifting fibre readout arrays to further

reduce the number of electronic channels required.

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If the fibre optic arrays are segmented then the global detection rate can be higherthan the local detector rate. However, this increases the number of PMT and readoutchannels required.

Question: What window thickness is likely to be required for a 320 mm x 320 mm

detector at the required pressure and therefore what is the likely size of the light spoton the readout array?

If the interpolation technique works well it may be possible to sum the outputs ofadjacent fibre optics and still achieve the required position resolution. This will resultin a reduction in the number of PMTs, ADCs and associated discriminator channels.

5. ISIS fibre coded readout

Fibre coded arrays could be used to readout the detector in a similar fashion to that

developed for the ZnS/6Li detectors at ISIS. With the ISIS system, optical collimationlimits the light from the scintillator to one pixel. For the GSD case either highstopping gas mixtures are required or a different means of processing the data has to

be developed. More of a limitation is the very high fibre density that is required forthis technique to succeed. In an SXD detector module with 4096 pixels, 16 348 fibresare required. If the same system is used on the GSD, the 128 mm x 128 mm detectorwould require 65 536 fibres and the 320 mm x 320 mm detector would require409 600 fibres!

Discussion of the various photon counting readout options

Table 1 shows the number of channels required for the different readout optionsdiscussed for the GSD.

Clearly for the Anger camera and position sensitive PMT type devices, this table issimply reflecting that the number of electronics channels is increasing in proportion tothe inverse square of the pixel dimension. The actual number of electronic channelsin any one system should not be taken too literally since there are always possibilitiesfor reducing the number of channels via indirect coupling methods etc. However, thetable does illustrate very effectively that without any readily available ASIC there is

considerable advantage in using the largest pixel size that allows the MILANDdetector characteristics to be realised. From this point of view this becomes one ofthe key questions for the GPS development programme to answer, since this willgovern both the cost and the performance of the device.

The LIP readout method offers enhanced position resolution, by providinginformation on both the position of the centroid of the reaction products in the gas andthe depth of interaction in the gas. While this could be a long term aim I believe it will

be more effective to characterise other features of the detector first in order to exploreother potential limitations before embarking on this aspect of the project.

128 mm x 128 mm 320 mm x 320 mm

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detector detector

Readout

option

PMT type No of

PMTs

No of

electronic

channels

No of

PMTs

No of

electronic

channels

LIP PMTreadout

Single anodePMTs

36 36 169 169

Anger

camera

readout

Single anode

PMTs

36 36 169 169

Anger

camera

readout

6 mm

pixelated

MA PMTs

9 576 49 576

Anger

camera

readout

3 mm

pixelated

MA PMTs

9 2304 49 12 544

Position

sensitive

PMT readout

4 256 16 1024

Wavelength

Shifting Fibre

readout

256 640

ISIS fibre

coded

readout

Too many fibres are needed

Table 1. An estimate of the number of PMTs and electronic channels associated withthe different readout options for the MILAND detectors.

An Anger camera type readout for the GSD, based on single anode PMTs is verysimilar to the LIP proposal except that in the interests of getting the project going inthe available time there will be no deliberate attempt to design circuitry to measurechanges in position as a function of the duration of the scintillator light from any oneevent. If in the course of this work it becomes possible to begin these measurementsthen this will be an added bonus and the information could be utilised at a later date

for the development of more ambitious readout electronics.

In looking first at the single ended PMT readout method the expectation is that lightcollection will be sufficient that recourse to the use of multi anode PMTs isunnecessary and at this time involves the construction of a large number of electronicchannels.

That a 30 mm photocathode is at least a realistic possibility can be inferred from theperformance of a Julios linear Anger camera. Such a camera, based on 6Li glassscintillator gives 1.2 mm resolution when using 8.6 mm x 24 mm PMTs, [ref 1].With a higher light output it should be possible to increase the dimension of the PMT

photocathode whilst maintaining the required position resolution. However, one

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problem is the gamma sensitivity associated with these devices which is ~ 10460Co,[ref 1]. An assessment of gamma sensitivity will therefore be crucial to the project.

To some extent the position sensitive PMT option (if I have understood it correctly)has a higher requirement in terms of the number of electronics channels than the

single anode PMT option. Since a 64 channel position sensitive readout system isalready available it would be interesting to try this out as a means of assessing the

performance of the GSD with a different pixel size.

The wavelength shifting fibre option is also of interest, and is something that is goingon at ISIS for the readout of solid state scintillators. However, current electronics isgeared to ZnS/6Li scintillator and this may not be so compatible with the light outputfrom GSDs. In addition, it is clear that the global and local counting rates will besimilar unless further coding partition strategies are adopted.

The coded fibre optic readout option looks unrealistic owing to the large number of

fibres involved.

How can CCLRC contribute most effectively to MILAND

It seems appropriate for LIP to pursue the detector development and in particular todetermine the optimum gas mixture to meet the requirements of MILAND.

CCD readout can be used to measure the performance of the detector, particularly interms of track length. Indeed, CCD readout must be of interest for those instrumentson reactor based sources where time of flight readout is not required.

Since ISIS is primarily interested in the development of event counting devices forToF analysis this is the logical area for CCLRC to contribute. The timescale for the

parallel development phase of MILAND is 18 months from the start of the project andwithin this time frame CCLRC has funding for 3 staff months.

Sugestion:

I suggest that in view of the available time we need to concentrate on just one of the

photon methods. My preference is for the development of an Anger camera systembased on single anode PMTs

In the first instance it will be necessary to determine the size, dynamic range andspatial and temporal spread of the signals coming from the detector.

At ISIS we have a 400 MHz dual channel digitiser. This has been used to record gasdetector signals from resistive wire detectors onto a PC. The signals can then beanalysed many times over, as a function of detector position and applying differentsoftware algorithms to derive optimum detector performance. Once the optimumconditions are determined the software algorithms can be fabricated in hardware.

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The development of Anger camera technology for GSD could benefit from thisapproach. We could purchase a second digitiser and set up 4 PMTs to look atscintillator light output. The hardware and software to do this could be tried out bylooking at the light output from a conventional scintillator and from LEDs.

There is a lot of expertise around in Anger camera technology both at Julich and inthe US, and the development of an Anger camera should be carried out with dueconsultation.

This set up could be developed virtually independent of the development of the GSDdetector and optimisation of the gas mixture.

Some simulation of the optimum shape and size of the PMTs will need to be carriedout. This is something that could be done at ISIS. We discussed at our meeting inJanuary, the possibility of a full scale simulation of the shapes of the different events,

but I think we all agreed that this is outside the frame work of ISIS. Such information

is crucial to the long term development of GSDs, but is quite time consuming. Itwould be a good subject for a PhD thesis.

Question: Francesco, it is not clear to me from the simulated plots that you showedwhat simulation capability you have at LIP and whether this an aspect of the projectthat you wish to pursue.

I think that the position sensitive PMT approach and the wavelength shifting fibrereadout are both reasonable options, but I do not believe we have time to pursue allthree options within the current project. I think that in the interests of gettingsomething done within the 18 month time frame of MILAND we have to be fairlylimited in our approach from the outset.

If you think this is a reasonable suggestion and division of the labour we should proceed to cost this and discuss how we could proceed to a 128 x 128 mmdemonstrator detector.

This approach has the advantage of allowing some development of the photoncounting electronics off line. Clearly we have to collaborate carefully in carrying outelectronics development on the detector.

ISIS will be able to provide some access to ToF instrumentation, but this time islikely to be in terms of days rather than weeks. Most of our development work iscarried out on an AmBe neutron source with brief visits to the accelerator as the needarises, until we are in a position for serious detector characterisation.

Conclusion

We propose that CCLRC should contribute to the development of a photon countingreadout system for the GSD.

Our preferred method of readout is via the development of Anger camera technology.

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We propose that CCLRC will initially perform some software simulations of theoptimum PMT dimensions and geometry if this is not already known and if thisagreeable to LIP.

CCLRC will then develop a four channel PMT readout cell. The PMTs will be

readout via two 2 channel 400 MHz digitisers.

Purchases will involve a second 2 channel 400 MHz digitiser, PMTs and accessories.

This cell will initially be set up and evaluated using LEDs and existing scintillators.

Once operational the cell can be used to record individual digitised signals from theGSD. Analysis of this data will be used to design appropriate electronics for a 128mm x 128 mm detector.

Whilst the design of the electronics for the 128 mm x 128 mm detector is within the

capability of CCLRC, it may be outside the 3 month remit of the CCLRC MILANDcontribution.

Reference

1. K.D. Muller et al., J. Neutron Res. 4 (1996) 135

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