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PHOTONUCLEAR EXCITATION FOR THE CALIBRATION OF PULSED BREMSSTRAHLUNG
SPECTRA
Y. Paiss, C. Eberhard, C. Collins
To cite this version:
Y. Paiss, C. Eberhard, C. Collins. PHOTONUCLEAR EXCITATION FOR THE CALIBRATION OF
PULSED BREMSSTRAHLUNG SPECTRA. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-
131-C9-133. �10.1051/jphyscol:1987922�. �jpa-00227340�
JOURNAL DE PHYSIQUE
Colloque C9, supplement au n012., Tome 48, decembre 1987
PHOTONUCLEAR EXCITATION FOR THE CALIBRATION OF PULSED BREMSSTRAHLUNG SPECTRA
Y. PAISS, C.D. EBERHARD and C.B. COLLINS
Center for Quantum Electronics, The University of Texas at Dallas,
POBox 830688, Richardson, TX 75083-0688,
U.S.A.Abstract: Many nuclear isomers were produced in the past by irradiating the appropriate ground state isotope with a broad spectrum high energy x-ray machine. It is proposed to use the known cross sections of the above photoexcitation processes to measure the spectral intensity of the high current pulsed x-ray machines with Mev end points. By using this technique each x-ray pulse can be analyzed separately.
Isotopes of Ir, Se, In, Br, Sr and Cd are shown to be suitable for spectral range ofA%dO keV - 1300 keV.
INTRODUCTION
There are several extremely high current pulsed x-ray machines with end point energies in the Mev region. These machines produce a few pulses per day with, generally, poor reproducibility. It is very difficult to measure the x-ray spectrum of every pulse, especially above a few hundred keV where methods like absorption,[l] Bragg reflection,[2] and Compton spectrometer[3]
are used.
The objective of this work is to demonstrate the use of resonance nuclear absorption to measure the photon flux at resonance energies. This allows the determination of the x-ray spectrum.
Resonance absorption by direct excitation from ground state into the metastable state is practically non-existent, since the absorption line width of any metastable level is extremely narrow. It is, however, possible to excite 'the metastable levels through an intermediate level (Fig. 1). It is proposed to measure the concentration of metastable nuclei formed in isotopes having long-lived metastable levels which can be populated by the decay of the state excited by nuclear resonance absorption. These measurements are performed away from the inducing source. In the energy range 0.3
-
1.3 Mev six nuclei are useful in determining the spectral density at six absorption energies possible (Fig. 2). There are some more nucle~[4] which can form metastable levels through the absorption in few levels (Fig. 3). The can be used in the smooth part of the spectrum (above the characteristic Tines of the machine) by numerical interpolation.EXPERIMENTAL
Samples of 191*1931r, 79~r, Il3Cd, 77~e, 1151n and 87Sr are placed where the spectral intensity is to be measured. The thicknesses of the foils are about 10 mg/cm2 which is thin enough not to degrade the spectrum by Compton scattering and which is "black" to the absorption of resonance lines, i.e.
,
few mean free paths for the Doppler-broadened Breit-Wigner absorption cross section. After the x-ray pulse, the foils are transported by a fast shuttle system to a calibrated HPGE detector and the photons from the decay of the metastable are counted. In the isotopes 1911r and ll3cd, the decay of the metastable is accompanied by the emission of electrons and x-rays and can be detected by counting the x-ray fluorescence (dissolution and liquid scintil- lation counting is considered).
The number of counts of each isotope with a single gateway level is proportional to the corresponding x-ray flux/unit energy on the foil. The proportionality constant is evaluated, taking into account the absorption in the foil, the resonance cross section (assuming a constant flux over the resonance energy width), the nuclear branching ratio, the metastable fluores- cence yield, the absorption of the fluorescence and the geometry and effi- ciency of the detector.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987922
C9-132 JOURNAL DE PHYSIQUE
An experimental calibration can be performed by using a continuous x-ray source with a known spectral intensity.
Another point on the spectrum is, of course, the machine end point energy. In the region where the spectrum is almost linear, (i. e
.
, on thedescending part) a simplified method can be used with multi-gateway resonance lines by calculating the center of qravity of all the lines and considering it as a single gateway isotope (e.g. 151n).
FIG. 1: Excitation of the metastable state through the decay of a single, intermediate excited state (gateway state).
1 Netastable State
f 0 t i r o d State
&
?IG. 2: The location of gateway states in e?.even isotopes ha.ving lonp-lived 1aetastab3.e states.
Wt@e Gatemy
bw-
m y convartad
-4 b o k ~ y
m e (iotamy
bw-
0 0 0 0 0 7 7 ~ e
0 0 0 0 o m ~ 9
0
0mln
00 om/n
r l r CPcd
@ 079~r o 8 7 ~ r 0 -AQ
I I I I I I 1 1 I I
# * I ~ ~ Q P P , ~ ~ P u + @
1 Enerqy of Gafeway Level ( k e ~ ) 1
r
&fe*oY States
rPtasWe Stote
1 t cl'mud State
FIG. 3: Excitation of the metastable state through the decay of several ictermediate excited states.
Acknowledgement: The authors gratefully acknowledge the support of this work by the Naval Research Laboratory with funds from the Innovative Science and Technology Program of the Strategic Defense Initiative Off ice.
REFERENCES
[I] GORCIES, STEVEN C., PEREIRA, NINO R., X-Ray Spectra from Various Flash X-Ray Simulators, Private Communication.
[2] VAN ASCHE, P. H. M. and KAERTS, E., GREEN, G. L. and DESLATTES, R. D., Crystal Relectivity for Bent Cystal Spectrometer, Paper Df. 1 , 1987 Spring Meeting APS.
[3] BALDWIN, GEORGE T. AND KEE, JAMES R.. IEEE Trans Nucl. Sci.. Vol. NS-33, 6 (December, 1986).
[4] YOSHIHARA, K., NEMETH, ZS., LAKOSI, L., VERES, A., Phys. Rev. C,
