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PROPERTIES OF A MINIATURE X-RAY SOURCE
M. Skowronek, P. Romeas
To cite this version:
M. Skowronek, P. Romeas. PROPERTIES OF A MINIATURE X-RAY SOURCE. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-111-C9-115. �10.1051/jphyscol:1987918�. �jpa-00227295�
PROPERTIES OF A MINIATURE X-RAY SOURCE
M. SKOWRONEK and P. ROMEAS
Laboratoire des Plasmas Denses, ~niversite Pierre et ~ a r i e Curie, T-12, E-5, 4, Place Jussieu, F-75252 Paris Cedex 05, France
Resume: Une source de rayons X, de petites dimenslons, ba* sur une dkharge dans le vide, a 6th etudib. Les caract6ristiques electriques mettent en Bvidence les pincements qui se produisent, mlme avec des courants de l'ordre du kA. Plusieurs sytemes ont ete e t u d i k 9 l'energie la plus faible de 30mJ a 500mJ, la d u r b d'impulsion X variait autour de 40ns ; avec une energie de l'ordre du joule les d u r h d'impulsion ont 6t6 obtenues de 15ns i3 5ns. Le spectre est 6tudi6 par la m6thode des Crans absorbants. La temperature du plasma varie de 3 keV a 7keV.
Abstract: A minlature X-ray source, based on a small vacuum discharge has been studfed. The electrical characteristics are given : evidence of pinches i s shown even with kA pulses. Few systems were studied: at the lowest energy, from 30mJ to SOOmJ,the X-ray pulse duration may vary about 40ns; with an energy of 1 J, the pulse duration can vary from 15ns to 5ns. The spectrum i s studied by the absorption method.
The plasma temperature vary from 3 keV to 7keV.
1.
lntroductlon
Vacuum discharges are well-known X-ray sources. "Hot spots" are formed during the pinch phase which may have microscopic dimensions. The energy of the emitted radiation lays i n a great range between soft X-rays and more than ten times the applied voltage. Thus, these devices are very useful i n plasma research, biology reactions studies, surface etching, ion spectroscopy and other applications.
As a consequence of the pinching, the tube voltage increases, which gives rise t o the emission of hard X-ray bursts[ll.
A vacuum discharge may produce a dense minute plasma, emitting X-ray whose energy increases as the current grows [21.
Near the peak value of the current, the quanta of the X-rays which come out have energies of several hundred keV f o r a discharge voltage of 10 to 20kV. Fukai and Clothiaux [31 have proposed a mechanism, based on the high resistivity of the plasma pinch due to the constrict ion and the strong turbulence.
In such a case, strong electric fields appear which accelerate the electror)s and ions to produce hard X-rays as energetic as 20 times the discharge potential. Cilliers e t al. [41 have deduced from a radiation enhancement a t
20p
the electron density t o besx
1 ~ * O c m - ~ and t h e temperatures about 1 OkeV. Fast time resolved measurements of the X-rays show that the emission i s due t o short bursts.Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987918
C9-112 JOURNAL DE PHYSIQUE
Burhenn et a1.[51 have measured two temperatures of the micro plasmas produced from the electrode material:
the average temperature was 8.3keV and the extreme temperature was higher than 200keV in the case of tungsten.
The lower temperatures scale as T = 42' (eV) and the higher as T .;0.32 z3.1(ev). Morita and Fujita [61 have carefully studied the spatial structure of the plasmas created in a vacuum spark. The plasma temperature was about 2.2keV and the electron density was of the order of 10''cm-~.
Time-integrated X-ray pinhole photographs show the existence of two different plasmas : "hot spots" of very small diameter and a more diffuse and extended plasma. High temperature plasmas are not always produced although the hot spots can be observed in every shot. A spatially resolved
K,
spectrum shows that the hot plasma (FeXXIV-XXVI) has a size of about 10pm.
The reproducibility of such vacuum spark X-ray sources has been treated by Wong and Lee [71
.
They have found that, for a laser-initiated vacuum spark w i t h an interelectrode distance of 5mm, the reproducibi 1 i t y i s better than 80%.We have summarized on table 1 the main characteristics of previous experimental works.
We present here a miniature X-ray source based on a vacuum spark, and we show that the above mentioned properties are generally the same for our source as for more important and powerful devices. In this paper, the stored energy was limited to the range between 20mJ to 1J w i t h X-ray pulses in the range 2ns-Sons.
In the following, f i r s t , we describe schematically our experimental set-up.
The results of the time resolved spectral study are then given, which are i n agreement w i t h some of the
Table I: Main characteristics of the different vacuum discharges.
U H F ) E (J) A T ( ~ s ) T(keV) Authors
0.1 20 100 - [ I 1
15 1500 - 100 121
30 1500 - - I41
30 1500 - 8.3 151
200 [51
28 3000 - 2.2 161
33 6000 - - [71
tnF)
2 1 20 6 [81
2.5 1 12-5 3 this
0.37 c0.5 40 3 paper
conclusions found previously in the 1 i terature.
2. Experimental set-up 181.
The vacuum discharge source i s a classical one: the anode i s a pointed rod, made either of molybdenum or tungsten;
the cathode i s a tube whose axis coincides w i t h that of the anode. The X-rays are emitted through the tube and are well collimated. In the present study , there i s no special triggering.
The external dimensions of the vessel chamber containing the discharge cell and the capacitor are 0 =50mm and I=100mm and i t s mass i s less than lkg.
I t i s connected to a small vacuum pump which ensures a residual pressure i n the discharge space about of 1Pa. Two discharges were studied : I ) the f i r s t i s produced by means of a high voltage cable of C=372pF, charged in the range 10kV-50kV which stores an energy in the range 20mJ-500mJ and delivers a 40ns pulse(source I); ii) the second one i s produced
,
uslng a capacitor of C=2500pF, charged from 15kV to 30kV which stores an energy in the range 0.33 - 1.1 J. I t delivers a X-ray pulse havingfollowing the interelectrode distance.
The intensity of the current i s recorded by means of a low-inductance shunt and a digital transient recorder.
The X-rays are measured by means of a rapid plastic scintillator (NE 102 A) coupled to a photomultiplier and also recorded on the transient recorder.
Different f o i l s were put i n the path of the X-rays, allowing a spectral analysis by the absorption method based on the Bremsstrahlung continuum [91 or on line emission[ 101.
3. Results
The discharge current i s not very reproducible and has a shape consisting of a single rectangular pulse, over which are superimposed one or two short spikes. These spikes, about IkA, correspond to strong current inversions and are likely due to the sausage pinch effect. I t i s interesting to note that such events occur w i t h such a low current
.
The X-ray emission i s comprised of one t o t w o pulses, being shorter than 20ns. In some experimental conditions, we have reached the time constant of our measuring device of about Ins. The emission waveform, while not exactly reproducible, shows considerable similarity from one shot to another.The time relations between the current, the voltage, and the X-ray emission were studied in order to discriminate between a classical diode behaviour and that of a pinch controlled source. It was d i f f i c u l t to obtain a definitive conclusion owing to the photomultiplier transit time j i t t e r and to some uncertainties i n the rise-time of the scintillator. At f i r s t sight, It seems that the principal photon
applied. The main characteristics of a classical diode emission are the continuum intensity given by:
Icont= K Z V ~ ( where Z the atomic number and V the voltage)
lri
our case, the variation of the spectrally integrated intensity w i t h the applied voltage is: I= k v 4 and i s independent of Z[a].
4.Spectral study
The different lines which can be emitted by the anode material are i n the range 2 5 k e ~ - IOkeV i n the case of W, and about 16.5kev or 2.5keV i n the case of Mo. The stainless steel cathode does not emit.
The absorption method uses three types of metals. Aluminium, Nickel and Gold The methods used to treat the absorption are based on t w o different assumptions:
i) the line absorption values are as given in
I
101 (L.A. method);ii) the absorption of the Bremsstrahlung continuum from plasma emission i s based on the calculations made by Elton [91 (B.C. method).
We have used successively: A1 f o i l s (thickness 6mm, 7mm, and 8mm), Ni f o i l s (thickness 15/ 1 OOmm, 20/ 100mm and 30/ 100mm) and Au f o i l s (thickness 40um, 45um and 50pm). On the table 2, are displayed, the different results found.
The temperatures found, by the B.C.
method for the source I i s 6 to 7 keV and f o r the sources I 1 and 111 350.5keV The line energy i s found generally about 30keV Nevertheless, in the case of a Bremsstralung continuum, as the varlation of the intensity w i t h the frequency Is exponentlal, the continuum
C9-114 JOURNAL DE PHYSIQUE
Table I I : Line energy and temperature determinations based on line absorption (L.A.) and Bremsstrahlung continuum ( B . 0
E(L.A. 1 T(B.C. 1
source I
- -
(keV) (keV1(0.2J-0.5J;=40ns) Voltage (kV)
35 2Q5 Q1
40 2855 6 i l
45 3 Q 5 6 . 2 l
5 0 27*5 7 . 2 1
source I1
( =IJ; 15ns)
2 0 2 9 5 3rt0.5
source 111
(-- 1 J; 5ns)
20 2Q3 3*0.5
source are very restricted and that the reproducibi 1 i t y i s acceptable. Some of the characteristics of that source are similar to those of much larger devices.
Acknowledgments
This work was supported by the Cet?tre National de la Recherche Scientifique. It was also under contract by the Direction des Recherches, Etudes e t Techniques.
We thank Professor C. Bonnelle, head of the Laboratoire de Chimie- Physique of the
P.
& M. Curie University, and R. Haug Directeur de Recherche, Laboratoire de Physique des Decharges, for their cooperat ion.may appear as a wide line, w i t h a maximum given by the relation [ I 1
I
:Emax
=
2 T E and T in keV In our case, the "line energy" i s about 30keV, much higher than 2T:6keV- 12keV.
These methods give unprecise results due t o the fact that the spectrum may be mixture of lines and Bremsstrahlung.
5. Conclusion
The purpose of our study was to obtain a small X-ray source, of short duration, whlch can be directed onto any Interesting object a t short distances (<5mm). This low energy X-ray source was designed to be light, cheap, and portable. We have measured the characteristics of the emission. A short pulse of high energy i s obtained which may be useful f o r plasma diagnostic purpose. We have shown that the extension and the excursion of the
References
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