LEPC detector system measured at the Xe L edge region

Nuclear Instruments and Methods in Physics Research A 367 (1995) 83-87

ELSEWIER

The response functions of the HEPC /LEPC detector system measured at the Xe L edge region C. Budtz-Jgrgensen”‘“,

C. Olesen”, H.W. Schnopper”, G. Ulmb

T. Ledererb, F. Scholzeb,

“Danish Space Research Institute, Gl. Lundtojievej 7, DK-2800 Lyngby, Denmark bPhysikalisch-Technische Bundesanstalt, Abbestr. 2- 12. IO587 Berlin, Germany

Abstract The Danish Space Research Institute will provide a set of two low energy proportional counters (LEPC) and two high energy proportional counters (HEPC) for the Russian SPECTRUM-X-Gamma mission. The detectors are based upon the technology of the microstrip gas chamber (MSGC). HEPC and LEPC use a Xe/CH, mixture as the counter gas. The response function as well as the pulse height-energy relation for a Xe/CH, filled MSGC were investigated with monochromatized synchrotron radiation of the BESSY double crystal monochromator. The detector response functions were recorded at -100 selected photon energies in the range from 1.8 to 5.9 keV. The response functions were measured with photon energy steps of 5 eV around the Xe L subshells. The average pulse height versus photon energy relation shows clear jumps at the L edges of 55 ev 25.6 eV and 153. eV at the L,, L, and L, shell, respectively. The widths of the pulse height distributions indicate an increase of the Fano factor for Xe at the L absorption edges. These results are in good agreement with earlier predictions. Detailed analysis of the shape of the photopeaks as well as of the tail and escape contributions will also be presented.

1. Introduction The Danish Space Research Institute will provide a set of four imaging counters as part of XSPECT, the Danish contribution to the SODART telescopes [l]. The prototypes are presently being tested and the flight models are under construction. The detectors employ the novel microstrip gas counter (MSGC) technology where the wire grids of the conventional proportional counters are replaced by narrowly spaced conducting microstrips (MS) which are accurately deposited (to.2 pm) on an insulating substrate. Oed [2] introduced this idea and demonstrated its qualities as a detector for low-energy alpha particles and protons. At DSRI, the MSGC has been adapted for use as an X-ray detector for astronomical applications [3]. As a result of this development work, DSRI will provide a set of four imaging MSGCs as part of XSPECT. A low and a high energy detector with energy ranges of 0.2-8 keV and 2-25 keV, respectively will be provided for each of the SODART telescopes. The feasibility study and development phase of the MSGCs for SODART have now been concluded. The design of the present MS plates will tolerate gas gains up to 3 X 104. It is possible, therefore, to * Corresponding author. E-mail [email protected] 0168~9002/9.5/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-9002(95)00570-6

detect X-ray energies down to -200eV and to maintain an adequate image quality. An energy resolution of K=WW, = 0.33(E/l keV)“* keV is achieved [4] which is at least as good as that obtained with a single wire proportional counter, although not sufficient to resolve many of the spectral features expected to be observed with XSPECT. Measured spectra will, therefore, be analyzed by folding trial input spectra with the detector response and comparing the calculated spectrum with the measured one. If the source statistics are sufficient, the best fit iteration of the trial spectrum with the model will be a good representation of the source spectrum. An accurate knowledge of the detector response is required. In this context it is of special importance to determine the response at the counter gas X-ray absorption edges where non-linearities of the response are expected to be present. Both HEPC and LEPC use Xe + 10% CH, and the present work was aimed at a detailed understanding of the MSGC response function, especially in the energy region of the Xe L absorption edges. Recent Monte Carlo simulations made at the University of Coimbra, Portugal, of the absorption of X-rays in Xe demonstrated that both the Fano factor, F, and the mean energy, w, required for the production of one primary electron show sharp increases at the Xe L edges [5,6]. Experimental evidence of these effects were earlier both

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C. Budtz-J$rgensenet al. / Nucl. Instr. and Meth. in Phys. Res. A 367 (1995) 83-87

found from spectra recorded with the EXOSAT Gas Scintillation Counter (GSPC) [7] as well a a later GSPC measurement employing radioactive sources [8]. Systematic experimental evidence is, however, still lacking. The present investigations were made at the BESSY synchrotron radiation facility, where a DSRI MSGC was illuminated with X-rays at -100 selected energy points in the energy range from 1800 to 5900 eV

2. Measurements and results

,

The measurements were performed on a beam line equipped with a double crystal monochromator [9] that allows X-ray energy selection with an accuracy of t2 eV The monochromator has a very low stray light level (
200

400 Channel

Nr.

Fig. 1. Measured response (+ ) of the MSGC to monochromatized photons (E = 4800 eV). The full line describes the best fit to the data.

the detector. The escape peak is also well characterized by the Prescott function (1). All the recorded pulse height spectra were analyzed in a similar manner and the fits were made using ADAPTION [l l] - a general least squares fitting computer code. Fig. 2 displays the dependence of the mean pulse height, m, as function of the energy of the monoenergetic X-ray photons. In the range from 4700 to 5500 eV, m is not proportional to the photon energy but shows discontinuities at the Xe L absorption edges. The straight lines shown in Fig. 2 are line fits to the data points in between the edges. From these fits, jumps of the mean pulse height corre-

2.1. Pulse height versus energy relation The measured pulse height distributions by Prescott functions [lo] of the form: N,,,,(c)

=

NW;;;

exp

were modeled

-cm”*; c”*)* ,

(1)

where the number, N, of counts per channel, c, is given by the mean value, m, and width parameter, Q, of the Prescott function. Fig. 1 shows the pulse height spectrum recorded at 4800 eV overlaid with the best fit model spectrum. The fit is excellent and is significantly better than obtainable with the usual Gaussian function which cannot reproduce the slight skewness of the measured distribution. The fit contains also a low energy tail contribution represented by a linear term multiplied by a step function from zero to m:

&,,,W = [a + PclO(m - c) , where a and p denote term, respectively. The spectrum shown just above the L, edge which fluorescent Xe

(2)

the offset and slope of the linear 300

in Fig. 1 was recorded at an energy and contains, therefore, events for La, ,a2 (4110 eV) photons escaped

hL_

4000

_I

4500

5000 Photon

Energy

5500

6000

[eV]

Fig. 2. Average pulse height, m, as function of the photon energy.

C. Budtz-J@rgensen et al. I Nucl. Instr. and Merit. in Phys. Res. A 367 (1995) 83-87

sponding to drops in the energy linearity of 55.222.OeV, 25.6+2.OeV and 15.5Z2.OeV at the L,, L, and L, shell, respectively, are derived. These values are in fairly good agreement with the calculations of Santos et al. [6] who reported jumps of 59 eV, 22 eV and 5 eV, respectively at the subshells. The measured mean pulse heights show a nearly linear dependence with energy in the range from 1800 to 47OOeV but with an offset such that the photon energy E, = 70.22 10 eV corresponds to zero mean pulse height. This agrees surprisingly well with the calculations of Santos et al. [8] for which the linear approximation in the range from 1200 to 4700eV yields a vertical offset of 69 eV Under the assumption that the gas gain is independent of the photon energy the quantity Elm is proportional to the mean energy, W, required to produce one electron-ion pair in the gas. Fig. 3 displays w(E) derived from the present data set and normalized to a value of 22.4eV at 5900eV [8]. The jumps at the Xe L subshell energies are clearly visible and the data confirms the overall, behaviour as given in Fig. 3 of Ref. [6]. The calculated data confirms the smooth decrease of w with energy in the range from 1800 to 4OOOeV.

2.2. The energy dependence of the Prescott width, Q Fig. 4 displays the energy dependence of the Prescott width parameter Q in units of eV as derived from the least squares analysis. Q is remarkably constant up to the energy of the L, subshell where it jumps from 10.4 eV to 11.1 eV. The quantity Q is related to the variance, ai, of the Rescott distribution through the simple relation: ui=2mQ.

85

88 1000

2000

3000

Photon

4000 Energy

5000

6000

[eV]

Fig. 4. The energy dependence of the Prescott width parameter Q. Left scale in eV units. right scale in dimensionless units. The conversion was made assuming w = 22.4 eV at 5.9 keV

The near constancy of Q confirms, therefore, the E “’ dependence usually found for the energy resolution of proportional counters, The significant jump at the Xe L edge has, to our knowledge, not been reported earlier for proportional counters. A sharp increase at the absorption edges of the Fano factor for Xe has, however, recently been predicted by Dias et al. (51, an effect which is explained by the fact that the energy transported by the photoelectrons decreases drastically at the shells. The authors predict a jump of the Fano factor from 0.2 to 0.3 at the L-shells. This jump will only partly affect the resolution of the MSGC since it is determined primarily by the avalanche statistics. The combination of the fluctuations of the number of primary electrons (fi = E/w) and the fluctuations due to the avalanche statistics is well known and can be expressed as [ 121:

(3)

(4) where u2 is the variance of the measured distribution, G the gas gain and ug the variance due to the avalanche

multiplication. Employing Fq. (3) and substituting A = m/G normalizing Q = Q/G one gets: 2riQ = riF + F_j

and, re-

,

or

(5)

Q=$+F,,,L where F,,, = u:jG’ is a constant. The observed jump of Q of 0.035 (see Fig. 4, right y-axis) corresponds, therefore, to a change of the Fano factor by 0.07, in fairly good agreement with the calculated value of 0.1 [5]. 22.0 * 1000

2000

3000 Photon

Fig. 3. The mean present data set.

ionization

4000 Energy

energy,

5000

6000

2.3. Tail-to-peak ratio

[eV]

w(E). obtained

from the

Fig, 5a displays the energy dependence of the content of the tail of the pulse height distribution relative to the

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86

C. Budtz-J@rgensen et al. I Nucl. Instr. and Meth. in Phys. Res. A 367 (1995) 83-87 lO.OF

0 ‘-7

L

P 0.030

.c

I Y x a 0.020

G

6

1.0 -

22

,o .L F

0

0.010

::

z

o.ooo~.. 1000

Jo.1

..I.‘,

2000

3000 Photon

4000 Energy

5000

If

6000

0.000~..be 4600

[eV]

4800

r

5000

I

5200

Photon

Fig. 6. Relative contribution 1.000

photon b)

.o

5400 Energy

5600

I

5800

6000

[eV]

of escape events as function

of the

energy.

measurements above the L threshold. These points lie slightly above the previous correlation, a behaviour which at present is not understood. i-

2.4. Escape peak contribution 0.001t, 0.1

1.0 Photon

ott.

length

10.0 [mm]

Fig. 5. (a) The energy dependence of the tail content relative to the content of the main peak (left scale). The dashed line represents the X-ray photon attenuation length in the counter gas (right scale.) (b) Scatter plot of the tail content versus the photon attenuation length. 0 represent data below the L, threshold. + represent data above the L, threshold.

of the main peak. The tail-to-peak ratio smoothly decreases with the photon energy, but also shows a sharp increase at the L absorption edges. It is obvious to correlate this behaviour with the attenuation length of the X-ray photon in the counter gas. This is shown as the dashed line in Fig. 5a. Incomplete charge collection can occur for events where the X-ray photon is stopped close to the window and loses some of the primary electrons to the window. As the energy is increased above the L, edge, photons are absorbed much closer, about a factor of 4, to the window and have consequently a much higher probability to lose electrons to the window. The correlation between the tail content and the photon attenuation length is further illustrated in Fig. 5b which is a double logarithmic scatter plot of these quantities. The open squares represent the data below the L threshold and they show a nearly linear relation between the tail content and the reciprocal attenuation length. For LEPC, with an energy range of 0.2-8 keV, this relation predicts a rather substantial tail contribution at lower energies, especially for energies just above the M,,, edge where the minimum attenuation length of -100 pm is reached. Extrapolating the observed dependence yields a tail contribution of 15% here. The data points shown as crosses in Fig. 5b represent content

The contribution of fluorescent escape events were analyzed by fitting the observed escape peaks taking the most prominent L transitions into account. Fig. 6 displays the energy dependence of the total escape yield relative to the content of the main peak. For energies just above the L, threshold, La, (41lOeV), La, (4096eV) and L,, (4719 eV) transitions give an escape yield of 1.7%; at the L, threshold, L,, (4416 eV) and L,, (5039 eV) transitions increase this to 2.2% and at the L, threshold, L,, (4509 eV) and L,, (4448 eV) transitions result in a further slight increase. These contributions, although rather small, will be taken into account in the analysis of in-orbitHEPULEPC observations.

3. Summary The response function of a Xe/CH, filled MSGC was measured in great detail in the X-ray photon energy range from 1800 to 5900 eV Earlier predictions [5,6] of sharp increases at the Xe L edges of both the mean ionization energy, w, and of the Fano factor, F, for Xe were confirmed by the present measurements. The pulse height distributions show low energy tail contributions which, although small in the present energy range, might be of importance for sub-keV photon energies. We are considering an extension of the present measurements down to the Xe M edge region.

References [I] H.W. Schnopper, SPIE Proc. 2279 (1994) 64. [2] A. Oed, Nucl. Instr. and Meth. A 263 (1988) 351.

C. Budtz-J@rgensen [3] C.

et al. I Nucl. Instr. and Meth. in Phys. Res. A 367 (1995) 83-87

Budtz-J#rgensen, M.M. Madsen, P. Jonason, H.W. Schnopper and A. Oed, SPIE Proc. 982 (1988) 152. [4] C. Budtz-JBrgmsen, Rev. Sci. Instr. 63 (1992) 648. (51 T.H.V.T. Dias, F.P. Santos, A.D. Stauffer and C.A.N. Conde, Nucl. Instr. and Meth. A 307 (1991) 341. [6] F.P. Santos, T.H.V.T. Dias, A.D. Stauffer and C.A.N. Conde, Nucl. In&. and Meth. A 307 (1991) 347. [7] A. Peacock, B.G. Taylor, N. White, T. Convoisier and G. Manzo, IEEE Trans. Nucl. Sci. NS-32 (1985) 108.

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[8] J.M.F. dos Santos, C.A.N. Conde and A.C.S.S.M. Bento, Nucl. Instr. and Meth. A 324 (1993) 611. [9] J. Feldhaus, F. Sclafers and W. Peatman, SPIE Proc. 733 (1980) 242. [lo] J.R. Prescott, Nucl. Instr. and Meth. 22 (1963) 256. [ 1l] Adaption, Brosa GmbH, Amiineburg, Germany. [12] D. West, Proc. Phys. Sot. 66A (1953) 306.

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