Development of Particle Detectorson the Base of Minsk Synthetic Monocrystalline Diamond

K.Afanaciev, M.Batouritski, V.Gilewsky, G.Gusakov,

I.Emeliantchik, A.Litomin, V.Shevtsov National Center of Particle and High Energy Physics

Belorussian State University

International School-SeminarThe Actual Problems of Microworld Physics

Belarus, GomelJuly 23 - August 3, 2007

Particle physics needs extremely radiation hard detectors

Expected annual dose: 10 MGy / year

Diamond is the only materialable to operate at such doses

ILC: e+e- collider

Initial phase 500 GeV, upgradeable to 1 TeV

BeamCal is the innermost system in the forward region and covers polar angle ranges of 5mrad to about 28 mrad.

Problems of polycrystalline CVD detectors:• high density and non-uniformity of structural defects due to polycrystalline nature• development of high excess currents with dose accumulation• strong dependence of response amplitude on radiation environment

Monocrystalline synthetic diamond is a much more promising material due toabsense of highly defective intercrystallite boundaries

Types of synthetic detector-grade diamond:• polycrystalline CVD• monocrystalline HPHT

Poly- or monocrystalline diamond?

Difficulties on the way towards a good diamond detector

Ideally diamond detector is just a solid state ionization chamber

In reality next problems exit:

• impurities limit the life time of free charge carriers• defective surface layer creates undesirable space charge• diamond-metal junction constitutes Shottki diod causing nonlinear field distribution

Following techniques can help to provide good detector quality:

• thermobaric processing converts nitrogen defects into less active aggregated state;• thermochemical surface procesiing removes defective layer and microcracks;• two-layer metallization with carbidized precoat provides ohmic contact

Monocrystalline particle detectorson the base of Minsk synthetic diamond

Parallel-sided plates cut out of crystals with mass 0.5 1.5 carat were used as initial samples.

Temperature gradient method in Fe-Co-C environment was implemented with help of“split sphere” apparatus.

Metallization was performed by thermal sputtering of titanium and gold with subsequent annealing for carbidization.

Clamping contact device was used for electric measurements.

Performance of the detector

UI 171s1

-1,00

-0,50

0,00

0,50

1,00

1,50

-500 -300 -100 100 300 500

U, V

I, pA

10mV/div

Subpicoampere currents prove good quality of metallization

Response to 90Sr -particles

Detectors on the base of thermobarically processed diamond Nitrogen is the main impurity in diamond

C-defects (nitrogen in replacing position)prevail in synthetic diamonds

Thermobaric processing allows to convertC-defects into (less harmful) A-defects

Types of defects are seen in IR absorbtion spectra: C-defect: 1135cm-1 A-defect: 1282cm-1

P = 6.7GPaT = 1800C t = 4 hours

Conditions of thermobaric processing:

Results of thermobaric processing

IR spectrum of the diamond detector before thermobaric processing

IR spectrum of the diamond detector after thermobaric processing

Response to -particles of diamond detector(sample 219) before thermobaric processing.

Response to -particles of diamond detector (sample 219) after thermobaric processing.

10mV/div 20mV/div

C-defect C-defect

A-defect

A-defect

Avalanche-like behavior of one of the samples

200mV/div

Response to 90Sr -particles

Response amplitude is much greater than expected,even greater than that of our reference diamond of De Beers production,where it’s close to theoretical limit.

Avalanche multiplication caused by charge gradient due to nitrogen content non-uniformity can be an explanation.

Development of avalanche diamond detector could be an interesting option,not researched so far.

Conclusions • Extremely radiation hard detectors are much needed currently, and synthetic diamond is able to satisfy this need;

• Synthetic monocrystalline diamond have advantages over commonly accepted CVD plates due to absense of highly defective intercrystallite boundaries;

• Tested samples proved that monocrystalline diamonds, produced with split sphere method, are able to work as particle detectors

• Low cost of crystals produced with this method together with high radiation hardness can provide wide range of applications for these detectors