1. Development and characterization of the Floating Gate Dosimeter for radiation dose measurements
PhD student: Matteo Brucoli - EN/STI/ECE
Supervisor: Dr. Salvatore Danzeca - CERN
Prof. Laurent Dusseau - UM2

2. Overview Temperature effect compensation Radiation monitoring at CERN
Total Ionizing Dose monitoring
Floating Gate Dosimeter: working principle
60Co calibration and characterization:
Sensitivity, linearity, resolution
Batch to batch variation, HDR and LDR tests
Temperature effect compensation
Mixed field irradiation:
FGDOS vs. RadFETs
Kdose measurement
Outlooks and Conclusions
Development and characterization of the Floating Gate Dosimeter for radiation dose measurements - Matteo Brucoli

3. Radiation Monitoring at CERN
Memory
Why is Radiation Monitoring so important?
Monitoring of the Radiation Level along LHC
Failure prevention of electronic equipment
Investigate the cause of failures
Installation, relocation, and shielding need to be designed according to radiation level
SEE DD
TID
PIN Diode
RadFet
How Radiation affects Electronics?
Cumulative Effects
Total Ionizing Dose (TID): deposit of ionizing energy, which leads to the change of electrical properties
Displacement Damage (DD): atom displacement caused by interactions of high energy particles with nuclei
Radiation Monitoring System - RadMON
Stochastic Effects
Single Event Effects (SEEs)
Development and characterization of the Floating Gate Dosimeter for radiation dose measurements - Matteo Brucoli

4. Definitions
Generally, dosimetry is the study of how radiation imparts energy to matter
More specifically, dosimetry is the measurement of absorbed dose in matter
Absorbed dose (Also often called Dose or Total Dose): the mean energy absorbed per unit mass of irradiated material
The SI unit of dose is the (Gy): 1 Gy = 1 J/kg
Another unit of dose used in the space world is the (rad) 1 rad = 100 ergs/g (1 Gy = 100 rad) The standard definition for the TID effect is a `circuit degradation or failure resulting from radiation-induced charge trapped in insulating layers (usually oxides)’
Development and characterization of the Floating Gate Dosimeter for radiation dose measurements - Matteo Brucoli

5. Total Ionizing Dose Monitoring at CERN
Vbias
RadFET is a special p-channel MOS with a thick oxide layer. The ionizing dose produces a charge accumulation in the SiO2 layer, which leads to shift of the threshold voltage (ΔVth)
Basic mechanisms:
e/h pair generation and recombination
Hopping transport
Oxide trapping → ΔVth,ox
Interface trapping → ΔVth,it
ΔVth = ΔVth,ox + ΔVth,it
Development and characterization of the Floating Gate Dosimeter for radiation dose measurements - Matteo Brucoli

6. RadFETs drawbaks ♦ Cumulated dose leads to:
Decreasing of trapping probability
Decreasing of the oxide electric field
The radiation response becomes non-linear
♦ Mixed Particle Fields influences RadFET Radiation Response
♦ Non linearity requires to be compensated by using a Calibration Curve
♦ Limited Resolution does not allow to measure low doses
♦ Annealing affects RadFETs measurement
400nm RadFET - Dose Rate = 300Gy/h
Development and characterization of the Floating Gate Dosimeter for radiation dose measurements - Matteo Brucoli

7. Floating Gate Dosimeter (FGDOS) – Working Principle
Buried gate is pre-charged by injecting holes → QFG ↑,VFG ↑
A fraction of the e-/h+ pairs generated by ionizing radiation are separated by the electric field in the oxide, surviving recombination
e- move toward the buried gate and discharge it → QFG ↓ , VFG ↓
The VFG variation is converted in current by the reading Mosfet
The drain current is converted to frequency by the current to frequency converter (I2F)
I2F FG
Floating Gate S D
Injector
gate ox.
Field oxide N+ N+
p Silicon B
Development and characterization of the Floating Gate Dosimeter for radiation dose measurements - Matteo Brucoli

8. Floating Gate Dosimeter (FGDOS) – Linearization
The Auto-recharge process allows to obtain an extremely linear response
The sensor is forced to work within the linear portion of the characteristic curve (~0.6 Gy)
The raw response profile has a saw tooth shape (Non Compensated Curve), whose jumps are compensated to achieve the real radiation response (Compensated Curve)
The charge injection is controlled by the TIDMON board
Whole Dynamic Range
Linear Dynamic Range
Compensated Linear Dynamic Range
The auto-recharge process allows to reach a linearity of 1% per 10Gy
Calibration Curve is substituted by a Calibration Factor, i.e. the sensor sensitivity [kHz/Gy]
Development and characterization of the Floating Gate Dosimeter for radiation dose measurements - Matteo Brucoli

9. 60Co calibration and characterization
Sample to sample variation has been evaluated → within ±2%
Batch to batch variation has been evaluated → within ±5%
High Dose Rate (3.2 Gy/h, HDR) and Low Dose Rate (0.32 Gy/h, LDR) → within ±2%
Sensitivity and resolution has been evaluated
Sensitivity
Resolution [mGy]
FGDOS
31.4 kHz/Gy
>2
RadFET +5V
7 mV/Gy 60
RadFET GND
2 mV/Gy
200
Development and characterization of the Floating Gate Dosimeter for radiation dose measurements - Matteo Brucoli

10. Temperature Effect Compensation
Output frequency depends on temperature variations
A MOSFET, identical to the one reading the FG capacitor, is integrated on the FGDOS chip in order to provide a reference frequency signal which is immune to TID
The temperature sensitivity has been characterized: Fref is constant at -135 kHz/◦C, while for Fout it changes depending on the working point
The temperature compensation is performed by means of a lookup table, which need as input the FREF and the Fout
I2F FG
REF
Reading
Development and characterization of the Floating Gate Dosimeter for radiation dose measurements - Matteo Brucoli

11. CHARM - K factor measure by means of FGDOS and RadFETs
At CHARM, different mixed fields can be generated by playing with three key parameters:
Test position p
Facility configuration c → Target (Cu, Al, AlH) → Shielding position (In/Out)
For each position and configuration, the field can be characterized by means of a calibration factor
𝐃𝐨𝐬𝐞 𝐩,𝐜 𝐏𝐎𝐓 = 𝐊 𝐝𝐨𝐬𝐞 𝐩,𝐜 [𝐆𝐲/𝐏𝐎𝐓]
𝐃𝐨𝐬𝐞 𝐩,𝐜 =𝐏𝐎𝐓 ∙ 𝐊 𝐝𝐨𝐬𝐞 𝐩,𝐜
The calibration factor is given by the ration of two cumulative physical quantities:
Dose, which can be measured by FGDOS or RadFET
Proton on Target, which is measured by the Secondary Emission Chamber
Development and characterization of the Floating Gate Dosimeter for radiation dose measurements - Matteo Brucoli

12. K factor stabilization
Experiment setup:
Position R10
Al target
Shielding Out
Dose rate ≈ 0.7 Gy/h
Resolution influences the k factor stability
Example: Kdose measured through a RadFET 0V read by RadMon V5
ADC resolution
Resolution RadMon V5 = 1 Gy
The dose to be cumulated to reach the stabilization is larger than 11 Gy
Development and characterization of the Floating Gate Dosimeter for radiation dose measurements - Matteo Brucoli

13. Mixed Radiation Field Experiments
Radiation tests were performed at CHARM in position R1 for different targets (Cupper, Aluminium, Aluminium sieve)
Thanks to the higher resolution, the Floating Gate Dosimeter allows to detect much lower doses
Experiment setup:
Position R1
Cu target
Shielding IN
Dose rate ≈ 0.3 Gy/h
Development and characterization of the Floating Gate Dosimeter for radiation dose measurements - Matteo Brucoli

14. K factor stabilization – Cumulated POT
The POT to be cumulated to achieve oscillations lower than ±5% of the Kdose is considered
Dosimeter
Kdose
[Gy/POT]
POT±5%
[POT]
FGDOS
5.63 · 10-16
0.61 · 10+14
RadFET +5V
5.73 · 10-16
5.15 · 10+14
RadFET 0V
5.74 · 10-16
8.15 · 10+14
Experiment setup:
Position R1
Cu target
Shielding IN
Dose rate ≈ 0.3 Gy/h
Development and characterization of the Floating Gate Dosimeter for radiation dose measurements - Matteo Brucoli

15. K factor stabilization - Summary
In order to quantify the velocity of stabilization, the POT to be cumulated to achieve oscillations lower than ±5% of the Kdose is considered
Target
Dosimeter
Kdose
[Gy/POT]
POT±5%
[POT]
ΔD±5% [mGy/POT]
ΔT±5% [h] Cu
FGDOS
5.63 · 10-16
0.61 · 10+14 35
1.5
RadFET +5V
5.73 · 10-16
5.15 · 10+14
300 12
RadFET 0V
5.74 · 10-16
8.15 · 10+14
470 20 Al
3.20 · 10-16
0.66 · 10+14 21
1.6
2.97 · 10-16
8.20 · 10+14
250
2.85 · 10-16
31.1 · 10+14
890 75
AlH
1.86 · 10-16
1.20 · 10+14 23 3
1.65 · 10-16
27.3 · 10+14
450 65
1.84 · 10-16
37.1 · 10+14
680 90 *
Maximum spread Kdose ~ 11%
POT±5% FG is (at least) 8 times lower than POT±5% RadFET +5V
POT±5% FG is (at least) 13 times lower than POT±5% RadFET 0V
LHC Arcs 0.3 Gy for 30 fb-1 →
Integrated Luminosity to obtain the calibration factor is
L±5% FG ~ 4 fb-1 ,
L±5% ±5% RadFET +5V ~ 32 fb-1
L±5% ±5% RadFET 0V ~ 50 fb-1
* Results presented in: Brucoli at al., “Floating Gate Dosimeter suitability for Accelerator-like Environments,” submitted to IEEE Trans. Nucl. Sci.
Development and characterization of the Floating Gate Dosimeter for radiation dose measurements - Matteo Brucoli

16. Outlooks and conclusions
FGDOS gamma ray characterization
Higher resolution (2 mGy for the FG, 60 mGy for the RadFET)
Very good linearity (1% per 10 Gy): Calibration Curve no needed
Batch-to-batch and Sample-to-sample variation within ±5%
High Dose Rate and Low Dose Rate variation within ±2%
Temperature effect has been compensated
Mixed field irradiation
Calibration factor measurement (FGDOS vs. RadFET)
POT±5% FG is (at least) 8 times lower than POT±5% RadFET +5V
Development and characterization of the Floating Gate Dosimeter for radiation dose measurements - Matteo Brucoli

17. Thank you!
(BACKUP Slides)

18. 60Co calibration and characterization
Radiation tests were performed for High Dose Rate (320 rad/h, HDR) and for Low Dose Rate (32 rad/h, LDR)
The variation from batch to batch has been evaluated
Sensitivity, linearity and resolution has been evaluated
Batch
Dose Rate
Max spread w.r.t. the average [%]
Batch comparison
HDR
1.9
LDR
3.7
Dose Rate comparison
Batch A
Batch B
9.5
Sensitivity
Resolution [mGy]
FGDOS
31.4 kHz/Gy 2
RadFET +5V
7 mV/Gy 60
RadFET GND
2 mV/Gy
200
Batch
Dose Rate
[Gy/h]
Spread among sensors CI(±1.96σ)
Max spread w.r.t. the average [%] A
3.2 – HDR
± 2.9
2.3
0.3 – LDR
± 3.2
2.7 B
± 4.0
2.9
0.3 - LDR
± 5.0
2.4
Development and characterization of the Floating Gate Dosimeter for radiation dose measurements - Matteo Brucoli