1 Research on Erratic Dark Currents in pCVD Diamonds Diamonds are Forever Adam Edwards, Stanford University M. Bruinsma (UC Irvine), P. Burchat (Stanford), H. Kagan (OSU), R. Kass (OSU), D. Kirkby (UC Irvine), B.A. Petersen (Stanford) RD42 Meeting, CERN May 14, 2004

2 Introduction n In February 2003, the 1.5T magnetic field was temporarily lost and erratic dark currents in two diamonds were observed.

3 Experimental Questions n Some of the questions that we have been trying to answer about these erratic dark currents u Are these surface/edge currents? u Is it an effect of the metalization? u Is it a property of the whole diamond or a localized effect? u Is this common to pCVD diamonds? u Are these currents caused by radiation? u How do they depend on bias voltage? u How does a magnetic field change them?

4 Reproducing These Effects n Erratic currents have been seen with every diamond sensor setup. n Erratic currents are always eliminated by a 1.5T magnetic field perpendicular to the electric field. Example: Diamond “UT31” with solder connections, kapton packaging, guard ring and CrAu metalization. Example: Diamond “UT31” placed in 1.5T

5 Reproducing These Effects n We’ve worked with various setups. u Different metalizations u Different metalization geometries, including guard rings u Different packaging, using wire bonds and soldering u Different DC readout electronics n These erratic dark currents are real and reproducible in the lab.

6 Laboratory Tests n We have recently used four new pCVD diamond sensors made with G10 packaging and wire bonding. u Two sensors have one pad and two have 4 pads u Two sensors have AuCr metalization and two have AuCr/Al u All sensors have a guard ring n CCD measured ~200μm u Exposed to 1kRad from 90 Sr for CCD measurement, then heat treated at 400°C n Irradiation tests performed with a ~11kRad/H on contact 60 Co source

7 Diamond Sensor Operation n Both with and without radiation and with no applied magnetic field, all four diamonds were operated at 100V without seeing any onset of erratic currents. No radiation or magnetic field Bias switched from 500V to 100V Radiation in Rad/Hour No magnetic field

8 High Voltage Stability Before Irradiation n Biased at +500V u One pad with 100pA  4000pA F (after 13 days with no erratic dark currents) u Two pads with ~100pA u 9 other pads and two guard rings showed no erratic dark currents n Biased at -500V u Two with ~200pA u One with ~20pA No magnet field Erratic currents arise in different pads independently

9 60 Co Irradiation Test n Signal currents become obscured by the onset of erratic currents. n Erratic currents appear in each sensor pad independently Rads per Hour of Exposure No magnet field

10 60 Co Irradiation Test n Sensors exposed to ~2kRad/Hour form 60 Co (~140kRad in total) n Within minutes there are erratic currents seen n Some currents are an order of magnitude higher than expected from the amount of radiation. n Every biased sensor in irradiation has some erratic current. No magnet field

11 Effects of 60 Co Irradiation with 500V Bias, B=0 n Before Irradiation n After Irradiation Note the different current scales!

12 Effects of 60 Co Irradiation Tests Without a Bias Voltage n One diamond sensor had no voltage bias during the irradiation and had no subsequent increase of erratic dark currents. n A later irradiation of this sensor with a bias voltage created subsequent erratic dark currents of 250pA-15000pA. No magnet field

13 Lowering the Bias Voltage n We have found the lowering the bias voltage can reduce the amount of erratic dark currents. n No diamond sensors see any erratic currents at a 100V bias. n The voltage where the erratic currents go away is unique to each pad. n It takes ~hours before erratic currents return at higher voltage. No magnet field

14 Magnetic Field Test n All four sensors were placed in a 1.5T magnetic field with the same orientation as the diamond sensors inside BaBar n Over 5 orders of magnitude, all pads with erratic dark currents showed that they could be stopped by the magnetic field.

15 Conclusions n We have reproduced, in the lab, the erratic dark currents and magnetic field effects that are seen in BaBar. n Guard rings show that the erratic dark currents are not surface currents. n The erratic dark currents have been seen with 3 different kinds of metalization. n Erratic dark currents are localized, affecting each pad differently. n Every pCVD diamond that we have tested has had erratic dark currents. n These currents arise under high bias voltage and increase during irradiation at high voltage. n Erratic dark currents are not present at low bias voltages. n Erratic dark currents are not present at 1.5T magnetic fields.