References for Neutron Irradiation Tolerance Testing for CSC electronics

Neutron flux and energy spectrum at the location of Cathode Strip Chamber

Neutron energy range
Worst radiation (KHz/cm2 )
Least radiation (KHz/cm2)
0~1 KeV
100
20
1~10 KeV
100
10
10-100 KeV
100
10
100 KeV~1 MeV
100
10
1~10 MeV
20
5
10~100 MeV
10
1
100 MeV~1 GeV
5
1
Total
450
50
Annual flux (1012/cm2)
7.0
0.75
1 MeV equivalent neutrons(1012/cm2.yr)
1.8
0.24
Annual Total Ionising Dose (Gy)
11
3.1

10 years of neutron fluence at CSC worst radiation location

2~20 MeV ~3x1012 neutron/cm2
> 20 MeV ~1.5x1012 neutron/cm2

* These numbers are crunched from ATLAS detector simulation and are approximate values. Another approach with formula found in "Review of Particle Physics" gives somewhat similar numbers.
** The worst radiation occurs in the lowermost area of the CSC; and the least radiation occurs in the outermost area of the CSC detector.
*** Assuming ATLAS will run for 180 days in a year.


Figure 1. The green curve shows the neutron energy spectrum for ATLAS. The red curve shows the energy spectrum of the neutrons coming from 20 MeV deuteron beam stopped by a Beryllium disk.

Electronics to be tested

Any silicon device will be effected in a radiation environment. For the CSC readout electronics, the following devices will be tested: (These chips are manufactured by Agilent Technology)
G-link data networking components HP part no. & link to description
transmitter HDMP 1022
receiver HDMP 1024
transceiver HFBR 5305
evaluation kit (transmitter and receiver on separate board) HDMP 102K

Neutron Radiation Mechanism

The de Broglie wavelength of neutrons is comparable to atomic spacing. So most of the neutrons pass
through materials by elastic scattering or diffraction. But when the neutron energy is greater than
an energy threshold,  nuclear reaction comes in and plays a very important role in the neutron material interaction. The neutron energy threshold for n + Si -> p + Al is roughly 2.8 MeV, and for n + Si -> (alpha) + Mg is around 4.2 MeV.  When a nuclear reaction occurs, the charged ion will create a pulse. If the pulse is big enough, it enables digital flip-flop to change state. This phenomenon is called "Single Event Upset (SEU)". Although nuclear reaction is the main cause for SEU, other mechanism  such as inelastic scattering could also induce SEU. In our CSC neutron irradiation testing, we are more concerned on the SEU rate of the G-Link connection under ATLAS running environment.

The SEU cross section vs the neutron energy has the following characteristic:

  1. For thermal neutrons, energy is lower than 2 MeV, basically no SEU.
  2. 2 < Eneutron < 20 MeV, the lower limit corresponds to the threshold of inelastic reactions. In this energy range, the SEU sensitivity to neutrons varies a lot.
  3. Eneutron > 20 MeV, protons and neutrons have the same probability to induce SEU. With energy greater than 20 MeV, protons pass the Coulomb barrier, therefore protons and neutrons behave the same. In this energy range, the SEU sensitivity to protons/neutrons has a flat distribution as a function of proton/neutron energy.

Figure 2. The SEU cross section for G-Link transmitter (Liquid Argon result) induced by neutron as a function of deuteron beam energy.

Error Categorization

 
SEU Single Event Upset A change of state or transient induced by an energetic particle in a device. This may occur in digital, analog and optical components or may have effects in surrounding interface circuitry. These are "soft" errors in that a reset or rewriting of the device causes normal behavior thereafter. For the CSC G-Link components, there could be two types of SEU depending on the location of the bit on the chip, the first kind is that one data bit is corrupted, in this case, the pulse height on one strip will be changed thus the position of the track will be shifted due to this data bit corruption; the second kind is that one control bit of the G-Link is corrupted, in this case, the G-Link will fail in locking on a frame to transfer, in other words, the link is down, all the data will be lost during down time.
MBU Multiple Bit Upset An event induced by a single energetic particle such as cosmic ray or proton that cause multiple upsets during its path through a device. Energetic neutron is unlikely to cause a MBU.
SHE Single Hard Error an SEU which causes a permanent change to the operation of a device.
SEL Single Event Latchup a condition which causes loss of device functionality due to a single event induced high current state. An SEL may or may not cause permanent device damage, it requires power strobing of the device to resume normal device operations.
*Note: this single event effect specification is taken from NASA database and only the category related to CSC readout electronics is listed. The categorization is still applicable to ATLAS. For a complete description of the single event effect, please visit http://flick.gsfc.nasa.gov/radhome/papers/seespec.htm

Experimental Setup

The radiation tolerance testing should be done under normal operational condition. A realistic simulation of the data flow for CSC ATLAS will be essential to measure the SEU rate. The neutron flux in this testing is many order of magnitude higher than in the ATLAS environment, therefore a pile-up effect has to be studied to extrapolate the SEU rate in ATLAS running environment.

Two possible test setups (conceptul design).

Radiation Standard

The ATLAS CSC will have 64 chambers in total. Each chamber has 4 layers, and each layer carries 192 precision strips and 48 transverse strips. The preamplifier and shaper (P/S) for each strip sends the waveform on the strip to a Switched Capacitor Array (SCA) continuously with a speed of 40 MHz at each beam crossing. The SCA will sample the waveform (4 or 5 samples per waveform) and store the information in its cells, each SCA takes in 12 P/S inputs. When a real physical event occurs, a triggering signal triggers ADC to read in averagely 4 (5?) waveform samples stored in the SCA for each strip and digitize them to 12 bits data. ATLAS will have trigger rate of 100 KHz. Then the output of 16 channels of ADCs are MUXed and transferred to off-chamber electronics through two G-Link connections for data processing.  Another G-link is dedicated for transfering SCA control data for each ASM board. There are 960 G-Link connections for the whole ATLAS CSC.

To reach a safe SEU rate for the CSC electronics, we make assumptions as shown in the following table and then set a standard for G-Link chips.
Assumption Parameter value Comment
CSC total in-efficiency 0.1% In-efficiency is defined as 100%-efficiency.
In-efficiency per G-Link 1.04E-6 960 G-Links for ATLAS CSC.
Link-down time 2 ms Up-stream and down stream, each direction takes 1 ms to recover.
No. of chips per link 2 transmitter and receiver.
Data dropped after re-connection 8 triggers 8 triggers worth of data will be dropped to flush the garbled charges due to the link -down.
Link-down probability per SEU 100% Although not every SEU will cause link-down, link-down certainly introduces more problem. A control bit upset will cause a link-down thus result in this particular G-Link dropping all the data during down time. A data bit upset will just result in the change of the pulse height on one strip thus may shift the position of the hit on this particular strip. To set a safe SEU rate, we assume the worst for each SEU occurance.
Acceptable SEU rate  one SEU per 32 minutes per chip Neutron flux at ATLAS CSC worst radiation area in 32 minutes will be 8.7x108 including neutrons of all energy.  A factor of 5 has to be included here to account for the uncertainty in the neutron flux simulation.
Acceptable SEL rate one SEL per 2.56E6 hours per chip Assuming each ATLAS run takes 2 hours and when SEL occurs on a particular G-Link channel, all the data on this channel will be lost during the run. Starting a new run will reset the G-link to resume its normal function.

Statistics discussion:  In the situation of not observing any SEU/SEL in a period of time T,  the probability function to describe it using Poisson statistics is P(0)=exp(-T/T0) where T0 is the average time for one SEU/SEL event (unknown here).  The probability of not observing any SEU/SEL will drop down to 10% in timeT=2.3T0 then we may inteprete it as the average time for one SEU/SEL to occur is greater than T/2.3 with 90% confidence level.

Probability for observing number of events in time 2.3T0

n
0
1
2
3
4
5
6
P(n) (%)
10
23
26.4
20.3
11.6
5.3
2
The average number of events expected is 2.3.
To measure T0 to a significance of 10 sigma, the number of occurances has to be greater than 100.

Note: The electronics have to sustain three types of irradiation tests to be qualified

  1. Single Event Effect on the Digital electronics with 10 years of neutron fluence for A) En > 20 MeV and B) 2 < En < 20 MeV.
  2. Displacement Demage with the total fluence of 10 years of 1 MeV equilavent neutrons. The main effect will be on the analog microelectronics for gain loss, leakage current increase and threshold shift etc.
  3. 10 years of Total Ionising Dose with 1 MeV photons.

Neutron Radiation Stations

  1. CERI: a cyclotron provides 20 MeV deuteron beam. The deuterons are stopped on a Be disk  producing neutron flux by stripping reactions.  The maximum energy a neutron can get is 20 MeV; the mean energy is 9 MeV. This source could provide a flux of 3x109 neutron/cm2/sec. One hour of irradiation will accumulate more than 10 years of ATLAS CSC neutron fluence.   Centre d'Etudes et de Recherchesé par Irradiation, Orleans France .  Related link: Liquid Argon-ATLAS.
  2. 88" cyclotron at Lawrence Berkeley Laboratory. This source can provide 5-55 MeV proton beam with a profile of up to 4" in diameter and the beam is uniform to about 5-10%. The cost of beam time for physics use could be free if the test schedule is flexible OR  a charge of $637/hr  could buy high priority beam time.  A chunk of minimum 8 hours has to be scheduled.  Contact: Peggy McMahan, Research Coordinator <p_mcmahan@lbl.gov>. A more detailed description of this source.
  3. Crocker Proton Cyclotron at University of California, Davis.  This source can provide 63 MeV proton beam and deliver a uniform flux over a diameter of 7 cm. The flux can be in the range 3x105--2x1010 protons/cm2/sec.   The cost of using this facility is $500/hour. Contact: Carlos Castaneda, <castaneda@crocker.ucdavis.edu> (530) 752-4228.
  4. Indiana University Cyclotron Facility . This source can provide 30-200 MeV proton beam with a flux in the range 1E6 to 1E11 p/cm2/sec. The beam size could be less than 2 cm to 7 cm in diameter. The uniformity variation is less than 30%. The cost of using this facility is $1000/hr  and no less than 16 hours will be scheduled.  Contact: Charles Foster <foster@iucf.indiana.edu>.
  5. Cyclotron in Svedberg Laboratory at Uppsala University, Sweden. The cyclotron provides  25-180 MeV proton beams and with a maximum flux of 108 protons/cm2/sec in a solid angle of 0.5 msr measured for a beam diameter of 7.5 cm. The flatness of flux over the beam profile is about 10%.  Mono-energetic neutrons  could also be obtained from nuclear reaction7Li(p,n)7Be. The neutron flux from this source is about 104 neutrons/cm2/sec. The cost of using this facility is free for pure scientific research. Contact: Per-Ulf Renberg <renberg@tsl.uu.se> A more detailed description of this neutron source .
  6. Tandem Van de Graaf Accelerator at Brookhaven National Lab provides proton and deuteron beams with a maximum energy of 29 MeV. Using the same method as CERI, TVDG could provide neutron source of E(max)=29 MeV and E(mean)=13 MeV. This source has a tight schedule at this moment.
  7. A list of neutron sources http://neutrons.ornl.gov/NSatHFIR/othern.html
Related links: Radiation Hard Electronics page at CERN by Martin Dentan.

Created by Yong Li of UCI on January 26, 2000.