neutron and x-ray dosimetry around sahand plasma focus
TRANSCRIPT
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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
32
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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35 7 8
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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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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
10
12
14
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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
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2 3 4 5 6 7 8
R (cm)
Do
se (
mG
y)
0 45 90 135 180 225 270
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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