Light guide dosimeters and loss monitors

M. Körfer, DESY Salzau 22.01.20031

Light guide dosimeters and loss monitors

H. Henschel, J. Kuhnhenn

Fraunhofer-Gesellschaft INT

M. Körfer, K. Wittenburg

DESY

F. Wulf

Hahn-Meitner-Institut HMI


Motivation

Sensor for continuous online-monitoring of • radiation sensitive components (Undulator, Electronics)• accelerator sections with spatial resolution• fast beam losses

Dosimetry in small and tiny spaces

Insensitive to electromagnetic fields

Maintenance free operation

Three different systems have been studied:


Light Guide Systems

Dosimeter Beam Loss

Powermeter

OTDR

Cerenkov Radiation Light Attenuation

Photomultiplier

Properties of fibers

MM GI FiberPhosphor-Doping Small annealing

MM SI FiberNo DopingLarge diameter

Detectors

Measurement

System


Dosimeter Fiber:Optical fibers show (wavelength-dependent) a nearly linear

increase of attenuation after irradiation with ionizing radiationindependent from dose-rate and -source


OTDR

Goal: Continuous Online-Measurement of the local dose (e.g. along the LINAC)

Radiation creates color-centers in fiber Measurement of the attenuation with OTDR

-- short Laser pulse enters the light guide-- Rayleigh Scattering reflects light back to detector-- Color centers absorb backscattered light-- Absorption is proportional to the dose


OTDR Measurement

OTDR: Tektronix TFP2A

Laser pulse duration: 3 ns (FWHM) at 850 nm local resolution 0.6 m

Optical Fiber : FiberCore N2900107GALinear attenuation up to 2000 Gy

TTF-fiber length < 300 m no laser pulse wideningdue to the fiber bandwidth the local resolution along the accelerator is constant

Dynamic range about >12 dB @ 850 nm limitation of the max. dose and length

Installation of fibers along the vacuum chamberto get fast results


Results - OTDR

Time period 17 Day’s in 11/2000

Beam direction from the right

dynamic range limits observation of max. dose and max. length noisy signal in the injector region (INJ)

Dose budget:

• Bunch compressor BC2 up to 450 Gy• temp. Beam-line ACC3 up to 200 Gy•above 180 m the noise starts•resolution < 3 Gy (noise limit)

Fibre Length [m]

120 140 160 180 200 220

Do

se

[G

y(S

iO2)

]

0

100

200

300

400

500

140 160 180 200 220Fibrelength /m

12

14

16

18

20dezilamroN

thgilrewop

B/d


Powermeter

Goal: Precise, sensitive and fast online-dosimetry

-- light from Laser LED enters the light guide -- continuous measurement of the light power (attenuation)-- position of sensor defines spatial resolution

Radiation creates color-centers in fiber Measurement of the attenuation with Powermeter

calibration


Powermeter Installation at TTF

• 5 - 20 Fiber windings around beam pipe • Dynamic range > 50 dB• Resolution: 140 mGy

Local Fibre Optic Radiation Sensor Systemfor the Undulator Magnets at TTF

Ethernet-box 100

ConcreteShielding

SwitchPCDESY

ComputerCentre

Data transfer to the FhG-INT

Newport 8800Optical Power Meter System

LEDSource

660 nm

1311

97

53

115

1 : 16Splitter

1412

108

64

216

Reference

1

16

UNDULATOR1

4

In

Out

Webserver

Beam

1

2

3

In

Out

5

6

7

10

9

8

UNDULATOR2

12

11

UNDULATOR3


Comparison with TLD Measurements

Deviation: mostly < 25 %

Reasons:• not same position• TLD Error 10-20 %

Smoothed accumulated dose (2001-11-26 to 2002-01-21)



Cerenkov Radiation

Goal: Fast beam loss monitor with spatial resolution

-- ionizing radiation creates Cerenkov light in ps-range-- Time difference between external bunch trigger and light pulse defines position -- Broadband detector (GHz), high sensitivity, low noise => PMT, APD-- Photon-wavelength 200 nm -700 nm (I ~ 1/3)-- Integral of light pulse is proportional to dose

ACCELERATOR

FIBER OPTICAL CABLEIRRADIATION

PHOTODETECTOR

TIME MEASUREMENT

INTEGRATOR

INTENSITY

POSITION

Ext. Trigger


ACC1ACC2 BC2DUMP Region Cerenkov-Experiment GUN

X

Y fiber

Cross-Section of beam pipe an fiber position

Cerenkov Radiation BLPM*

Goal: Fast transversal & long. beam loss profile monitoring

* Beam loss position monitor


Experiment at TTF1


Experiment at TTF1


Cerenkov Radiation BLPM*

TTF2 Prototype Undulatorkammer

Goal: Fast transversal & long. beam loss profile in the TTF2 Undulator

4 fibers close to beam

x

Y

fibers groove

* Beam loss position monitor


Summary

Measurment-System:

OTDR Powermeter Cerenkov

Application Online local dose detection by averaging

Online local dose detection in real time

Online local dose detection in real time

Comment No single bunch resolution

No single bunch resolution

Single bunch resolution in a train

Detector response A few minutes A few ms A few ns

Range of dose [Gy] 3 – 450 limited by OTDR

0.14- 2000 limited by fiber

Not calculated yet

Range of wavelength [nm]

850 - 1330 860 200 – 850

Resolution length 1m 0.01 m 0.2 m

reasonable Fibre length*

5km 0.01 m 1km

* depending on max. dose and position resolution


Preliminary Test at TTF Undulator

Pulse-width (=spatial resolution) defined by

1) shower2) light-dispersion3) Electronic (PMT)

Zoom

2 loss position per bunch



Regeneration for fibers with temperature

Goal: Maintenance-free long-term use

Temperature 250 C 90 % regeneration within < 5 min

Indu

ced

Loss

[dB

/km

]

Time [s]

First irradiation

Second irradiation,after "regeneration"


Simulation and reconstruction of losses


References

FIBRE OPTICAL RADIATION SENSING SYSTEM FOR TESLA H. Henschel, Fraunhofer-INT, Euskirchen, Germany, M. Körfer, DESY, Hamburg, Germany, F.Wulf, HMI, Berlin, Germany 5th European Workshop on Diagnostics and Beam Instrumentation, DIPAC, Grenoble 2001

Faseroptische Strahlundgssensoren fur TESLAGrundlagen und Abschlusbericht zum Forschungs- und Entwicklungsvertrag "Glasfaserdosimetrie fur TTF"H. Hensche, O. Kohn, S. Metzger, U. WeilandFraunhofer-Institut fur Naturwissenschaftlich-Technische Trendanalysen (INT)Interner Bericht 4/01, August 2001

H. Henschel, O. Köhn, M. Körfer, Th. Stegmann, F. Wulf Preliminary Trials with Optical Fiber Dosimeters at TTF TESLA-Report No. 2000-25 August 2000

H. Henschel, M. Körfer, K. Wittenburg, F. Wulf Fiber Optic Radiation Sensing Systems for TESLA TESLA Report No. 2000-26 September 2000

Optical Fiber Dosimetry at the Tesla Test Facility (TTF)H. Henschel, O.Köhn , M. Körfer , T. Stegmann , K. Wittenburg , F. Wulf 9th Beam Instrumentation Workshop Royal Sonesta Hotel Boston, 5 Cambridge Parkway, Cambridge, MA USA, May 8-11, 2000

Documents

Light guide dosimeters and loss monitors