Neutron and X-Ray Dosimetry Around Sahand Plasma Focus

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NTUNeutron and X-ray dosimetry around Sahand plasma focus

by:

S.Sobhanian, M.Golalikhani, M.A.Mohammadi and E.Ghareshabani

Department of Atomic & Molecular Physics,

University of Tabriz, Tabriz, Iran

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Sahand plasma focus

I – stage of break-down along the insulator and plasma-current sheath formation

II – stage of PCS radial acceleration

III – stage of radial compression and dense plasma focus formation

Plasma Parameters

Hot ~ 0.3- 2 KeV

Dense 1024- 1026 m-3

Lifetime ~100ns

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Sahand plasma focus

Sahand characteristics

Anode diameter: 50 cm

Cathode diameter: 76 cm

Insulator diameter: 48 cm

Total capacitor bank: 288 µF

Maximum stored energy: 90 kJ

Maximum charging voltage: 25 kV

Maximum current: ~ 1.1 MA

Working gas pressure:

0.1- 5 Torr

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X-ray production

•Bremsstruhlung

From energetic electrons and ions

•Line radiation

From de-excitation of atoms

X-ray in plasma focus

Soft (E<10 keV)

And Hard (E>10 keV)

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Neutron production

Plasma focus is a cheap pulsed neutron source

Mechanisms

2 2 1 3

1 1 0 1D D n He

Beam- target interaction

Fusion

10- 20% in the maximum compression period of plasma column

90- 80% in the decaying period

Experimental yield formula:

Ip: Plasma current

Ex: In Sahand Ip= 1MA y ~ 109 neutrons

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py I

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X-rays detection systems

For soft X-ray radiation: Semiconductor detector

SPPD 11-02 type PIN diode

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2

35 7 8

9

6

11

11 11

11

4 10

a)

1-vacuum-tight socket2-isolating flange3-device frame4-transitional flange5-SPPD-detector frame6-sensitive element7-filter8-mesh

9-diaphragm slit10-vacuum lock11-vacuum seals

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X-rays detection systems

For hard X-ray component: vacuum photo-diode

1 - scintillator2 - vacuum photocell bulb Ф-113 - detector frame4 – coaxial socket5 – anode Ф-116 – cathode Ф-11

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X-rays detection systems

For time – resolved measurements of hard X-ray and neutron we used fast NaI scintillator (5 ns) with photomultiplier

1 – plastic scintillator2 – detector frame3 – lead shield4 – photomultiplier bulb5 – voltage divider;

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Neutron detection

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816

15

9

10

2

6

5 1 15

14

13

12

11

77

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1 – discharge chamber (cathode)

2 – anode

3 – porcelain insulator

4 – anode insert

5 – central window

6 – spark gap switch

7 – capacitor bank cables

8 – semiconductor X-ray detector (SD)

9 – detector of neutron yield (G-M

counter)

10 – detector of hard X-ray radiation

(vacuum diode with scintillator, VD)

11 – pin-hole camera

12 – magnetic probe

13 – Rogowsky coil

14 – detector of neutron and X-ray fluxes

(PM with plastic scintillator)

15 – diagnostic ports

16 – vacuum locks

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Neutron detection

For time-resolved measurement of neutrons and HXR twoPMs coupled to NE-102 plastic scintillators with 5nsresolution (position 14) (located at 7m from plasma column)

Scintillation detectors register both neutrons and HXR withsame efficiency.

The integral neutron yield is measured with an activationdetectors (position 9) consisting of a self-extinguishingG-M countor surrounded by a silver foil of 0.1mm thicklocated inside a polyethylene moderator PM-13amplfication factor>106 (Cssh photocathode and 12dynode)

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Neutron detection

Typical signal from PM

X-ray: ToF= 20ns then v=3*1010(cm/s)

Neutron: ToF= 35ns then v=1.96*109(cm/s)

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Neutron detection

GM

Silver activation cross sections

Ag107(51%)= 45 barns

Ag109(49%)= 113 barns

Neutron detectors are calibrated by Pu(α,n) Be source with aflux of ~ 106n/cm2.s

Typical results: for 16 kV 1.8*109 neutron

and for 18kV 2.67*109 neutron

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Definitions

Unit of absorbed dose for any type of radiation

1 Gy (gray)=1 Joul/kg

1 rad = 100 erg/gr

1 Gy = 100 rad

Dose equivalent: amount of any type of radiationthat when absorbed in a biological system, resultsin the same effect

Dose equivalent H=DQ

Q increases with linear energy transfer (LET) in SIconvention. H is measured in Sievert

1 Sievert (Sr) = D.Q= (Gy).Q

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X-ray dosimeters

TLD (Thermoluminescence Detector)

A class of inorganic crystals. The inorganic scintillationmaterials when exposed to ionizing radiation, emit lightin the form of prompt fluorescence.

Luminesence:

•Flurescence: the excited electron de-excites promptly

•Phospherescence: the excited electron de-excites with somedelay

•Thermoluminescence: in TLD the stored energy is released byabsorbing thermal energy.

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Thermoluminescence materials

Some TLD are crystals to which a small concentration of impurityhas been added as an activator

Caso4:Mn , LiF:mgTi, CaF2:Mn

Li2 B4 O7 : Mn (TLD800), Zeff=7.42

LiF: MgCuP (commercial name: TLD100H

CaF2 (Zeff=1.63), CaSo4 Zeff is high (forenvironmental use)

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Neutron Dosimeters

Thermal neutrons TLD

Fast neutrons polycarbonate films

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Classification of neutrons

•Thermal neutrons (energy~ 0.25 eV)

•Intermediate neutrons 0.5-10 keV

•Fast neutrons 10keV- 10 MeV

•Relativistic neutrons > 10MeV

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Thermal neutrons

LiF in 3 forms

Natural LiF Harshaw TLD 100

Enriched 6LiF Harshaw TLD 600

Enriched 7Li Harshaw TLD 700

The most part of thermoluminescence comes from αparticles produced by neutron incidence: 6Li(n,α) 3H

6 1 3Li+n α(2.07MeV)+T (2.74MeV)

TLD responses to fast neutrons are generally smallcompared to and X rays.

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Fast neutrons

Heavy charged particles create defects in the form of tracks inpassing through materials like some organic crystals, mica, glass asplastic materials.

Films made of polycarbonate (CR 39 film) are siutable for thispurpose

LiB dosimeters are used in this method.

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X-ray dosimetry around Sahand PF

TLD (LiF crystal)

Uniform response independent of energy

(from soft X-rays to high energy rays

The condition of reading:

Tmax= 250 0C, H.R=6 0C/s Repeat 4times to measure the errors

ECCi=

ECC is elemental correction coefficient. N is number ofdosimeters TLi is reader’s response C

alibration coefficient is obtained from the slope of TL responsesversus doses.

1

.

N

i

i

i

TL

N TL

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Isotropy investigations

For isotropy investigations TLD are placed in acircle around the cathode part of PF at angles:

0, 90, 135, 180, 225, 270 and 315 degrees.

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R (cm) Angle Dose (mGy) R (cm) Angle Dose (mGy) R (cm) Angle Dose (mGy)

0 0 21.02 5 0 17.53 7.5 0 12.52

3 0 18.16 5 45 16.31 7.5 45 10.69

3 45 18.20 5 90 15.69 7.5 90 10.89

3 90 17.56 5 135 16.26 7.5 135 11.84

3 135 16.58 5 180 14.67 7.5 180 12.54

3 180 18.41 5 225 18.48 7.5 225 14.22

3 225 18.85 5 270 20.27 7.5 270 14.05

3 270 20.01 5 315 18.65 7.5 315 13.91

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R (cm) A.Dose(mGy)

0 21.02

3 18.25

5 17.23

7.5 12.58

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12

14

16

18

20

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0 1 2 3 4 5 6 7 8

R (cm)

Do

se

(m

Gy

)

The average dose for each distances

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10

12

14

16

18

20

22

2 3 4 5 6 7 8

R (cm)

Do

se (

mG

y)

0 45 90 135 180 225 270

25Control room

120 cm

220 cm

200 cm

80 cm

310 cm

Operator

Operator

Dose measurements in PF lab

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Dose measurements in PF lab

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Measurement of doses at 2 successive days

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Neutron dosimetry in Sahand PF lab

Dosimeter: polycarbonate film

Response appears as tracks

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Conclusion

•The pinch does not occur in the center

•Doses are in background level

•Most neutrons are produced by beam target interaction

•No considerable neuron dose absorption for people workingin the lab

•Some recommendations for safety against X-rays

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Thank You

and

Have a nice day