june 14, 2012dean schamberger1 anomalous hv currents in the d0 central calorimeter a.k.a. malter...

June 14, 2012 Dean Schamberger 1

Anomalous HV Currents in the D0 Central Calorimeter

a.k.a. Malter Currentsa.k.a. Malter Currents


Outline• What information do we have

– Lots: too much show if all

• What effect does it have on data– Systematic change in the detector

response

• Can we explain what we see?– With some hand waving arguments

agreement with many of the observed effects can be explained


Malter Process

• ions accumulate on oxide layer

• field extracts electrons from base metal increasing current

• oxide eventually breaks down and discharges surface

3


D0 readout Cell


Is U Oxide like Al Oxide?

• Resistivity of Al Oxide is 1 e14 ohm cm

• Resistivity of U Oxide at 83 K is 1.4 e16 ohm cm

– From D0 note 1154 by M. Lynn Stevenson (1991)

• Similar enough to cause Malter Currents


Excerpt from D0note780, RLM Nov,1988

• Current then about 25 times higher than Uranium decay rate would predict.


HV Current Monitoring• Both Test Beam and Run I HV current

monitoring data is no longer available• Starting in about 2002 “5 minute” HV currents

sampling on the 32 CC HV supplies is available• Occasionally in 2010-2012 high sampling rate

data (> 1Hz) was taken for more detailed studies• In the fall of 2008 one HV supply was split into

the 16 individual wires to “fix” the Purple haze noise. This allows use to study the current drawn from both ends of the same HV gang


CC Currents – no beam


ECN currents – no beam


ECS Currents – no beam


CC current – with beam


EC currents – with beam


CC HV “phi symmetry”


Malter Breakdown


End of store Current drop


Resistance MeasurementsMeasurements (plot) are forten surfaces in parallel

Table lists the single surface Resistance

Expected ~0.4 from the NimArticle description

Now understand we should have expected ~1.5 +/- 25%

Resistance essentially unchanged from when we built the detector


Current from the 2 ends


Changes for the better?


HV sag at twice the “typical” current draw at 3x1032

• Profile assumes uniform current per unit area

• Max voltage change is ~400 Volts

• Converting from voltage drop to signal decrease has never been measured


Long Term Behavior• The upper right plot is the current

for one channel over a 10 year period.

– The current increases by ~4

– The maximum increase is ~7 for channel 0

• The lower right plot is one year of running for an EC channel

– The jumps in the blue trace are thought to be resistive shorts which come and go

– There is no long term trend


CC HV Current continues rising

• Since 2002 the “no beam” current in the CC has risen by about a factor of 4

% raise in current


HV Currents (cont)

• In the 7 months following the last long machine downtime the current rose an additional 40%, with typical currents 175 Amp


HV Currents (cont)

• In the next 6 months only rose an additional 10%, with typical currents 200 Amp– This is about a factor of three less than the previous 6 months

Percentage


Current vs Luminosity• Current increase is

proportional to delivered luminosity

• All channels have a similar behavior

• Two points at L~7 and 12 are 2 measurements during the the same shutdown

– indicates the size of the error bars


Turn on Current

• The current draw in the CC is very different from the EC

• The CC (upper plot) takes ~2 days to reach equilibrium current while the EC is less than 5 minutes

• Note the very good exponential fit to the CC data

60.3 1 e 2.32t CC current draw. Red curve is fit

EC current. The plot covers 2 days and the current goes to full scale in one 5 minute sample period


EC-CC difference• Most likely difference between the EC and CC is in

the Uranium plates

• The EC had the UO2 removed with a high pressure water jet before assembly while nothing was done to the CC plates

• The readout plates were processed in the same manner for both detectors

– Shape is different (long-thin in CC, more square in EC)

• The UO2 coating can explain the unusual CC behavior


Other Features• After about 1 day some channels

break into stable oscillations

• The oscillations persist in a store

– period decreases

• Sharp drop at 11.3 hours is end of store

– 2 hours between store with 1 oscillation

– Vertical bar is losses from scraping

– Then current climbs with new store


Other Channels

• Other channels have a more complex pattern

• Plot on right shows 2 frequencies

• By using a numerical derivative technique one can do a frequency analysis on these channels


Frequency Analysis• Least Squares fit a

parabola to 100 consecutive data points

• Analytic derivative at center of fitted region

• Rolling fit over entire data set

• Averages out small data fluctuations

Derivative for HV Chan 0

Time in minutes

29


Channel 25

• Pattern suggests 2 different frequencies

• Can extend this method to more complex distributions Time in minutes

Derivative for HV Chan 25


Channel 0 Data

No store

Store 7261


Oscillation Cycle Times

• Cycle times are an hour or more

• Cycle times in a store increase by ~50% over the store

• No store cycle times are longer and constant


Change with Time

• Six channels had oscillations in 2002

• 22 had oscillations in 2011

• Channels in 2002 had one or two frequencies

• Almost all channels in 2011 have multiple frequencies


Beam On/Beam Off Slope

• Slope (increase in current/day) is much larger during shutdowns than during beam.

• Plots on right are the slope between two shutdowns and the average of the two slopes in the shutdowns

Blue: No beam slope in µA/Day

Pink: Average slope during stores in µA/Day


Change in Current with HV off

• Took 24 days of data with HV on at end of Tev.

• Then turned the first 16 channels off for 60 days and then back on for 18 days

• Following plot deletes the 60 day off period and plots the data as if the HV was always on

– a few day overlap is put in to help guide the eye.

• If the current continued to increase, we would expect the second part of the plot to be higher than the first


• The red curve is current after the HV was turned back on

• The green curve is the approximate average increase in current experienced by the 16 channels that were left on

• If current were increasing during the HV off period, we would expect the turn on to exceed the green line for some of the channels

• All pre turn off data is at or above the turn on data

• Conclude that there is no increase in current with HV off


Malter Breakdown

• Lower fig. shows breakdown detail

• FWHM of peak is ~200 s

• Downward slope time is ~70 sec

• RC time constant of HV supply is ~10 s

‣ Not a supply affect (see below)

• Neutralizing surface charge takes longer than bringing the charge through the oxide

– Little E field in transverse direction so charge movement is very slow


Start Of Store

Scraping structure supports RC of ~10 s

(power supply resolution)


Exponential Turn on Curve

• DC resistivity of UO2 can also explain this

• Model the U LAr system as a capacitor with the UO2 as the dielectric.

• Model as a circuit with a capacitor in parallel with a resistor

– Current increases as the capacitor charges up


Analysis

V q

CdV

dtdq

dt

1

CinetC

inet k V

Rk = current from Ar ions

V/R = ohmic leakage current through UO2

dV

dt k

V

R

1

C

V kR 1 et

CR

Solution with V=0 at t=0

Equation is the same as R in parallel with C


• let I = total current through oxide and f= fraction that reduces voltage on the oxide layer

• Fraction of current into the Argon is then

• The argon current is what we measure

fI i I k

f1 e

t

CR

iArgon 1 f kf

1 et

CR

iArgon 1 f I


Current Increase

• Linear current increase with no beam is just Ohm’s law

– UO2 must have a decreasing resistance– Decrease must be reduced when beam is

present

• Most likely due to semi conductor properties of UO2


• 1.3 volt band gap - similar to silicon

• Conduction is NOT the same as silicon

• No Hall effect observed

Semi Conductor Properties


No Hall voltage means

that current is notconducted by free

carriers


Polarons

• Charge transfer is by charge carrier hopping from one donor atom to the next.

• UO2 is p type so charge carrier is a hole

• As electron moves through the oxide, it polarizes the other atoms in the material creating a retarding field which slows down the electron.

• Combined affect is called a polaron


Polaron Theory

• Small polaron theory gives the conductivity as

– T= temperature, E=activation energy, k=Boltzman’s constant

• Plot of σT versus 1/T on Log x axis should be a straight line

– P. Nagels et al,Solid State Communications 1 35-40 (1963)

1

Te E /kT


Conductivity VS oxygen• Obeys polaron theory for 8 orders

of magnitude

– Data to 90 Kelvin

• Conductivity is strong function of oxygen content

– Change from UO2.0000 to U2.0064 (0.3%) changes the conductivity by 5357 (Nagels’s paper)

– We need only a factor of ~5

– Linear extrapolation would change the O content by ~6 parts per million


O2 in LAr

• Measured concentration at end of Tev is 131 parts/billion

• Current increase is a function of L and not time so O is likely made by the beam

– Candidate is the resistive coat which is epoxy based

– Basic epoxy bond is C O


Model

• Argon ions build up on the surface of the UO2 from beam or from U238 decay products

• Voltage increases with time which increases the current flow with time

• Some regions with oxide layers that are thin or sufficiently conductive build up enough voltage to cause Malter breakdown


Model cont• O is released into the LAr by the beam

• O is ionized by radiation and drifts to the UO2 surface and is absorbed decreasing the resistivity

– HV off stops this process and this is confirmed by the data

– Uranium decays keeps the diffusion process going with no beam so this explains the continued increase in current

• This must stop as the O is removed from the LAr but we did not wait long enough to observe this


Slower Current Increase with Beam

• This is just due to the radiation damage in the UO2 semiconductor.

– Similar to silicon– Generate deep acceptor sites which trap

electrons and thus increase the effective resistivity


Still to do

• Make an estimate of the ionization of 131 parts/billion in LAr to see if it is the right order of magnitude to support this hypothesis

– D0note 780 (R. McCarthy) quotes NWA test beam measurement of 42.5 nAmp/m2

from Uranium decays immediately after HV applied


Summary/Conclusions• Two types of Malter current effects

– Continuous– Breakdown

• Most of the CC Cal currents are likely caused by UO2 left on the surface of the plates

• Increase in current is likely from O continuing to contaminate the UO2

• Beam slows down the rate of increase• Increase only occurs when HV is on.• HV sag after 10 fb-1 of running is significant (even

at low luminosity) but no measurement on how large the effect will be.


Supplemental


CC EM HV “gang” definition

12

34

5

67

8

9

june 14, 2012dean schamberger1 anomalous hv currents in the d0 central calorimeter a.k.a. malter...

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