- Email: [email protected]
Application of ultrasound for the simultaneous improvement intensity and resolution of the Bonse–Hart diffractometer
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 529 (2004) 152–156
Application of ultrasound for the simultaneous improvement intensity and resolution of the Bonse–Hart diffractometer E. Iolina,*, L. Rusevicha, M. Stroblb, W. Treimerb, P. Mikulac a
Institute of Physical Energetic, 21 Aizkraukles St., LV-1006 Riga, Latvia b Hahn-Meitner Institute, Glienicker Str. 100, D-14109 Berlin, Germany c Nuclear Physics Institute, 25068 Rez near Prague, Czech Republic
Abstract It is calculated that simultaneous ultrasonic excitation of the Bonse–Hart monochromator and analyzer leads to the improvement of resolution and increasing of the intensity. It is argued that incoherent ultrasonic field is applicable in similar experiments. Ultrasonic phonon satellites were studied by means of Bonse–Hart channel cut crystal. Strong improvement of contrast with moderate loss of intensity was observed in this case. Double Crystal diffractometer with ultrasonically excited monochromator and analyzer was applied for the ultrasonic phonon satellite research. It was observed for the first time that this ‘‘double sound’’ technique leads to the strong improvement of contrast and simultaneously 60% increasing of the intensity. r 2004 Elsevier B.V. All rights reserved. PACS: 43.35.+d; 61.12.q Keywords: Ultrasound; Double crystal; Bonse–Hart; Neutron diffractometer
1. Introduction It is well known that Double Crystal (DCD) and Bonse–Hart (BHD) diffractometers are used for the neutron ultra small angle scattering (USANS) research of the numerous materials with characteristic size equal to several micrometers. However these instruments application is limited by the insufficient beam intensity even at the high-flux reactors. It is known that the total intensity, Iht, of the diffracted neutron beam can be increased by *Corresponding author. Fax: +371-7550839. E-mail address: [email protected] (E. Iolin).
means of ultrasonic excitation of perfect single crystal (s.c.) monochromator or analyzer. Unfortunately this Iht increasing is attended by the loss of the impulse resolution. It was supposed almost 15 years before to excite simultaneously high frequency ultrasound with the same frequency and non-controlled phase in DCD monocromator and analyzer and to increase Iht without loss (and even with an improvement) of resolution [1]. The ultrasonic effect is defined mainly by the value of the parameter N=|HW| where H is vector of scattering and W is h.f. ultrasonic wave amplitude. Ultrasonic satellites exist as additional zones of the almost total
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.04.196
ARTICLE IN PRESS E. Iolin et al. / Nuclear Instruments and Methods in Physics Research A 529 (2004) 152–156
reflections for the case of symmetrical Bragg scattering. The number of phonon satellites is B2N and width of each is roughly BDk0/N1/2 for the case of Nc1. Dk0 is Darwin plateau width. Each satellite is narrow, but they are numerous. The whole effective neutron beam phase space volume is increased by ultrasound that has led to the upgrade of the diffractometer parameters. However the first attempt to observe this effect was not successful, we observed increasing of the intensity and loss of DCD resolution [2]. We here calculated rocking curve (RC) for the case of simultaneous ultrasonically excited silicon Bonse–Hart triple-bounces channel-cut monochromator and analyzer. The calculated RC contains many peaks. The main peak intensity is increased almost 3 times and RC FWHM is equal only B50% of the ordinary BH RC FWHM. We argue that coherence (one-mode) ultrasonic excitation of BHD monochromator and analyzer is not necessary in our case. It seems especially important taking into account complicated geometry of BHD. At last we show the first experimental results [3]. We excited h.f. ultrasonic wave in Si s.c. plate and observed ultrasonic phonon satellites by means of BHD analyzer. The observed phonon satellite contrast was much better than for the case of DCD and intensity loss was quite moderate. We fulfilled first measurements when ultrasonic waves with the same frequency were simultaneously excited in DCD monochromator and analyzer. We found that peak intensity was upgraded up to 60% and simultaneously ultrasonic phonon satellite contrast was strongly increasing in accordance with theoretical prediction [1].
2. The main theoretical and experimental results In the frame of the dynamical theory the absorption (emission) of the ultrasonic phonons leads to the increasing of the neutron momentum at the value of dq ¼ n
ðk0 þ kh Þ oS 7nkS ; jk0 þ kh j v cosðYB Þ
n ¼ 71; 2; ::: ð1Þ
153
where k0, kh are the neutron momentum corresponding to the incident and diffracted beam, respectively; v is speed of neutron; oS, kS are the ultrasonic wave frequency and wave-vector. The last term of dq will be omitted in the following. It is the exact statement for the case of the transversal acoustic wave (TAW) spreading perpendicular to the plane of scattering and is a very good approximation for the case of cold neutrons used in our measurements. We limit ourselves by the case when dqcDk0, when neutrons are jumping resonantly between Dispersion Surface (DS) branches (see for example Ref. [4]). These resonance transitions lead to the existence of the new zone of the total reflection at the reflectivity R for the case of symmetrical Bragg reflection. The width of the total reflection zone for the n-phonon transition G(n) and position of the zone center qn are defined by the expressions: oS : ð2Þ GðnÞ ¼ Dk0 jJn ðHWÞj; qðnÞ ¼ n 2v cosðYB Þ Let us consider the case of multi bounces Bonse– Hart monochromator (or analyzer) excited by ultrasound. Tails of reflection (outside of total reflection zones) will be suppressed in such device so that the reflectivity R could be written in the form: RðyÞE
n¼N X
HðJn ðHWÞ2 ðy yðnÞÞ2 Þ;
n¼N
yðnÞ ¼ 2qðnÞ=Dk0 ; Hðxo0Þ ¼ 0
Hðx > 0Þ ¼ 1; ð3Þ
Jm(HW) is the Bessel function of the first order. R(y) contains many narrow peaks. The height of the each peak is equal to 1. BHD RC is defined by the self-convolution of R(y) (Fig. 1) for the case of ultrasonically excited monochromator and analyzer and contains many sharp peaks. It is seen from comparison with classical Bonse–Hart RC that the main peak intensity is increased 2.9 times, this peak is narrow, corresponding FWHM is equal only B50% of the ordinary BH RC FWHM. It seems that this can be applied for the neutron scattering research in the limit of USANS and probably for improvement of the neutron imaging for the case of the refraction contrast. The
ARTICLE IN PRESS E. Iolin et al. / Nuclear Instruments and Methods in Physics Research A 529 (2004) 152–156
154
the case of strong incoherent ultrasonic field and found that S(t) is always non-negative. We would like to calculate reflectivity RI(y) of the BHD monochromator and analyzer in the frame of Abragam model. Above discussed total reflection zones (2) are happened due to the resonance coupling between neutron states at the different DS branches. Corresponding coupling parameter is equal to the value of Jn(HW)2. Therefore the value of RI(y) can be obtained from R(y) (3) after substitution Jn ðHWÞ2 ) Jn ðHWÞ2
6 HW=3 HW=0
Intensity
5 4 3 2 1 0 -10
-5
0 Y
5
10
Fig. 1. BH diffractometer RCs HW=3, y(1)=5. Only RC central part is shown at the figure.
discussed (and observed—see following) whole RC is deeply modulated. This circumstance probably allows extracting sample scattering from the result of measurements of RC for the system: ultrasonically excited DCD or BHD and sample. Above discussed calculations were done for the case of the coherent (one mode) ultrasonic wave. However in our case the value of ultrasonic wave length lSB30–50 mm, Si plate thickness B5 mm. Therefore it is difficult to excite one-mode coherent ultrasonic field. What will be happened for the case of the incoherent (multimode) ultrasonic field? The similar problem was analyzed many years before by Abragam [5] for the case of Mossbauer effect with ultrasonically excited Fe57 source. Abragam considered completely chaotic ultrasonic field model with ultrasonic amplitude Gaussian distribution function f ðwÞ ¼ 2 expðw2 =w20 Þw=w20 ; Z N w20 ¼ w2 f ðwÞw2 dw
ð4Þ
0
and successfully interpreted experimental results. We applied Abragam model for the analysis of our Neutron Spin Echo (NSE) experimental data [6]. We observed inelastic neutron scattering by ultrasound and found that NSE signal S(t) is non-negative even for the case of strong excitation. S(t) signal should be negative for the case of strong coherent ultrasonic wave. We calculated S(t) for
¼ exp ððHW0 Þ2 =2ÞIm ððHW0 Þ2 =2Þ ð5Þ where Im is the Bessel function of the second order. Therefore it is not necessary to create one-mode ultrasonic field in the DCD or BHD; we have a chance to avoid these difficult problems solution and upgrade diffractometer parameters even for the case of incoherent ultrasonic waves. We received some experimental data concerning research of inelastic neutron scattering by ultrasound by means of DC and BH diffractometers (instrument V12a, BENSC, HMI, Berlin). We used cold, slow neutrons with wavelength 0.523 nm and studied symmetrical Si(1 1 1) Bragg reflection. Longitudinal (LAW) and transversal (TAW) ultrasonic waves with frequency 70.27 MHz were excited by means of LiNbO3 transducers in perfect silicon s.c. plates. In the first series of measurements LAW was excited in Si(1 1 1) plate. Neutrons diffracted at plate were analyzed by means of Si(1 1 1) plate (DCD regime) or triple bounces Si(1 1 1) BHD analyzer (BHD regime). Our calculation predicted that BH analyzer should lead to the strong improvement of contrast with moderate loss of intensity of the main peak (Fig. 2). It happened due to the strong suppression of reflectivity outside of the Darwin plateau in the BH analyzer. The shape of the observed RC (Fig. 3) confirmed this prediction. BH analyzer ‘‘allows’’ to observe higher order ultrasonic phonon satellites. We found some results concerning DCD with simultaneously ultrasonically excited monochromator and analyser. The last term of dq (1) could be missed due to the application of cold neutron
ARTICLE IN PRESS E. Iolin et al. / Nuclear Instruments and Methods in Physics Research A 529 (2004) 152–156
2.5
155
80000
BH-analyz. DC
2
sound in analyz. two sounds
70000
1.5
Count
Intensity
60000
1
50000 40000 30000
0.5
20000 10000
0 -10
-5
0 Y
5
10
0 -50
-40
-30
-20
-10
0
10
20
30
40
50
Ang. sec.
Fig. 2. RCs calculations. Neutron scattering by ultrasonically excites silicon plate for the case of triple-bounces Bonse–Hart (red line) and DC monochromator. HW=1. Only one-phonon satellites are taken into account.
Fig. 4. DCD RCs. Monochromator: black—ultrasound switch off; red—LAW, 70.27 MHz switch on. TAW, 70.27 MHz is excited in analyzer in all cases.
3. Summary 50000
BH analyz. DC
Count
40000
30000
20000
10000
0 -50
-40
-30
-20
-10
0
10
20
30
40
50
Ang. sec.
Fig. 3. The observed RCs for the case of DC (black) and BH (red, in electronic version of paper) analyzer. Monochromator—Si plate excited by LAW, 70.27 MHz. Calculated (1) first satellite position is equal to 10.9 ang. sec.
and effects of the TAW and LAW are similar. We excited LAW in monochromator and TAW in analyzer and used two different generators with almost the same generated signal frequency, 70.27 MHz. Therefore corresponding ultrasonic excitations were not coherent with each other. It can be seen from Fig. 4 that peak intensity is seriously, B60%, increasing and simultaneously the ultrasonic phonon satellite contrast is strongly improved in this ‘‘double-sound’’ case.
We suppose to excite simultaneously high frequency ultrasound with the same frequency and non-controlled phase in DCD and BHD monochromator and analyzer and to increase intensity without loss (and even with an improvement) of resolution. This approach is confirmed by the result of corresponding calculations. It is not necessary to excite coherent (one-mode) ultrasonic wave in such measurements; incoherent ultrasonic field is applicable. We received first positive experimental results concerning ultrasonic phonon satellites research: (a) BH analyzer allows successfully observe these satellites; BH analyzer leads to the strong improvement of contrast with moderate, B20%, loss of intensity of the main peak. (b) We found some results concerning DC diffractometer with simultaneously ultrasonically excited monochromator (LAW) and analyzer (TAW). We observed that peak intensity increasing up to 60% and simultaneously ultrasonic phonon satellites contrast was strongly improved in this ‘‘double-sound’’ case. Application of cold neutrons allowed minimising some problems connected with small deformation created in perfect silicon crystal by cooplant between ultrasonic transducers and s.c.
ARTICLE IN PRESS 156
E. Iolin et al. / Nuclear Instruments and Methods in Physics Research A 529 (2004) 152–156
At our level of our knowledge both above described results were observed for the first time. It seems that ultrasonic method can be applied for the neutron scattering research in the limit of USANS and probably for improvement of the neutron imaging for the case of the refraction contrast. The observed RC (Fig. 4) is deeply modulated. This circumstance probably allows extracting sample scattering from the result of USANS measurements at the BHD with ultrasonically excited monochromator and analyzer. It seems interesting application of ultrasound for the case of the Spallation Neutron Source (SNS), generating short-wave neutrons. BH Timeof-Flight diffractometer manufactured from Si(1 1 1) plates could use simultaneously neutron harmonics with wavelength l, l/3, l/5. The same ultrasonic wave will be more effective for the case of short wave neutrons. The number of ultrasonic phonon satellites HW(l/5)=5HW(l), HW(l/3)=3HW(l). Therefore multi-phonon scat-
tering will be easily excited for the case of short wave neutrons and could be observed by means of TOF technique. At last time—amplitude modulation of the ultrasonic wave allows separately excited ultrasonic effects for neutrons with wavelength l, l/3, l/5.
References [1] E.M. Iolin, Piz’ma Zh. Techn. Fiz. 15 (1989) 52 (in Russian). [2] E.M. Iolin, L.L. Rusevich, M. Vrana, P. Mikula, P. Lukas, Phys. Status Solidi B 195 (1996) 21. [3] E. Iolin, L. Rusevich, M. Strobl, W. Treimer, P. Mikula, BENSC Experimental Report, Proposal PHY-04-884, 19/ 12/2003. [4] E. Iolin, B. Farago, F. Mezei, E. Raitman, L. Rusevich, SPIE 3239 (1997) 406. [5] A. Abragam, C. R. Acad. Sci. (France) 250 (1960) 4334. [6] E. Iolin, B. Farago, F. Mezei, E. Raitman, L. Rusevich, Physica B 241–243 (1998) 1213.
Nuclear Instruments and Methods in Physics Research A 529 (2004) 152–156
Application of ultrasound for the simultaneous improvement intensity and resolution of the Bonse–Hart diffractometer E. Iolina,*, L. Rusevicha, M. Stroblb, W. Treimerb, P. Mikulac a
Institute of Physical Energetic, 21 Aizkraukles St., LV-1006 Riga, Latvia b Hahn-Meitner Institute, Glienicker Str. 100, D-14109 Berlin, Germany c Nuclear Physics Institute, 25068 Rez near Prague, Czech Republic
Abstract It is calculated that simultaneous ultrasonic excitation of the Bonse–Hart monochromator and analyzer leads to the improvement of resolution and increasing of the intensity. It is argued that incoherent ultrasonic field is applicable in similar experiments. Ultrasonic phonon satellites were studied by means of Bonse–Hart channel cut crystal. Strong improvement of contrast with moderate loss of intensity was observed in this case. Double Crystal diffractometer with ultrasonically excited monochromator and analyzer was applied for the ultrasonic phonon satellite research. It was observed for the first time that this ‘‘double sound’’ technique leads to the strong improvement of contrast and simultaneously 60% increasing of the intensity. r 2004 Elsevier B.V. All rights reserved. PACS: 43.35.+d; 61.12.q Keywords: Ultrasound; Double crystal; Bonse–Hart; Neutron diffractometer
1. Introduction It is well known that Double Crystal (DCD) and Bonse–Hart (BHD) diffractometers are used for the neutron ultra small angle scattering (USANS) research of the numerous materials with characteristic size equal to several micrometers. However these instruments application is limited by the insufficient beam intensity even at the high-flux reactors. It is known that the total intensity, Iht, of the diffracted neutron beam can be increased by *Corresponding author. Fax: +371-7550839. E-mail address: [email protected] (E. Iolin).
means of ultrasonic excitation of perfect single crystal (s.c.) monochromator or analyzer. Unfortunately this Iht increasing is attended by the loss of the impulse resolution. It was supposed almost 15 years before to excite simultaneously high frequency ultrasound with the same frequency and non-controlled phase in DCD monocromator and analyzer and to increase Iht without loss (and even with an improvement) of resolution [1]. The ultrasonic effect is defined mainly by the value of the parameter N=|HW| where H is vector of scattering and W is h.f. ultrasonic wave amplitude. Ultrasonic satellites exist as additional zones of the almost total
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.04.196
ARTICLE IN PRESS E. Iolin et al. / Nuclear Instruments and Methods in Physics Research A 529 (2004) 152–156
reflections for the case of symmetrical Bragg scattering. The number of phonon satellites is B2N and width of each is roughly BDk0/N1/2 for the case of Nc1. Dk0 is Darwin plateau width. Each satellite is narrow, but they are numerous. The whole effective neutron beam phase space volume is increased by ultrasound that has led to the upgrade of the diffractometer parameters. However the first attempt to observe this effect was not successful, we observed increasing of the intensity and loss of DCD resolution [2]. We here calculated rocking curve (RC) for the case of simultaneous ultrasonically excited silicon Bonse–Hart triple-bounces channel-cut monochromator and analyzer. The calculated RC contains many peaks. The main peak intensity is increased almost 3 times and RC FWHM is equal only B50% of the ordinary BH RC FWHM. We argue that coherence (one-mode) ultrasonic excitation of BHD monochromator and analyzer is not necessary in our case. It seems especially important taking into account complicated geometry of BHD. At last we show the first experimental results [3]. We excited h.f. ultrasonic wave in Si s.c. plate and observed ultrasonic phonon satellites by means of BHD analyzer. The observed phonon satellite contrast was much better than for the case of DCD and intensity loss was quite moderate. We fulfilled first measurements when ultrasonic waves with the same frequency were simultaneously excited in DCD monochromator and analyzer. We found that peak intensity was upgraded up to 60% and simultaneously ultrasonic phonon satellite contrast was strongly increasing in accordance with theoretical prediction [1].
2. The main theoretical and experimental results In the frame of the dynamical theory the absorption (emission) of the ultrasonic phonons leads to the increasing of the neutron momentum at the value of dq ¼ n
ðk0 þ kh Þ oS 7nkS ; jk0 þ kh j v cosðYB Þ
n ¼ 71; 2; ::: ð1Þ
153
where k0, kh are the neutron momentum corresponding to the incident and diffracted beam, respectively; v is speed of neutron; oS, kS are the ultrasonic wave frequency and wave-vector. The last term of dq will be omitted in the following. It is the exact statement for the case of the transversal acoustic wave (TAW) spreading perpendicular to the plane of scattering and is a very good approximation for the case of cold neutrons used in our measurements. We limit ourselves by the case when dqcDk0, when neutrons are jumping resonantly between Dispersion Surface (DS) branches (see for example Ref. [4]). These resonance transitions lead to the existence of the new zone of the total reflection at the reflectivity R for the case of symmetrical Bragg reflection. The width of the total reflection zone for the n-phonon transition G(n) and position of the zone center qn are defined by the expressions: oS : ð2Þ GðnÞ ¼ Dk0 jJn ðHWÞj; qðnÞ ¼ n 2v cosðYB Þ Let us consider the case of multi bounces Bonse– Hart monochromator (or analyzer) excited by ultrasound. Tails of reflection (outside of total reflection zones) will be suppressed in such device so that the reflectivity R could be written in the form: RðyÞE
n¼N X
HðJn ðHWÞ2 ðy yðnÞÞ2 Þ;
n¼N
yðnÞ ¼ 2qðnÞ=Dk0 ; Hðxo0Þ ¼ 0
Hðx > 0Þ ¼ 1; ð3Þ
Jm(HW) is the Bessel function of the first order. R(y) contains many narrow peaks. The height of the each peak is equal to 1. BHD RC is defined by the self-convolution of R(y) (Fig. 1) for the case of ultrasonically excited monochromator and analyzer and contains many sharp peaks. It is seen from comparison with classical Bonse–Hart RC that the main peak intensity is increased 2.9 times, this peak is narrow, corresponding FWHM is equal only B50% of the ordinary BH RC FWHM. It seems that this can be applied for the neutron scattering research in the limit of USANS and probably for improvement of the neutron imaging for the case of the refraction contrast. The
ARTICLE IN PRESS E. Iolin et al. / Nuclear Instruments and Methods in Physics Research A 529 (2004) 152–156
154
the case of strong incoherent ultrasonic field and found that S(t) is always non-negative. We would like to calculate reflectivity RI(y) of the BHD monochromator and analyzer in the frame of Abragam model. Above discussed total reflection zones (2) are happened due to the resonance coupling between neutron states at the different DS branches. Corresponding coupling parameter is equal to the value of Jn(HW)2. Therefore the value of RI(y) can be obtained from R(y) (3) after substitution Jn ðHWÞ2 ) Jn ðHWÞ2
6 HW=3 HW=0
Intensity
5 4 3 2 1 0 -10
-5
0 Y
5
10
Fig. 1. BH diffractometer RCs HW=3, y(1)=5. Only RC central part is shown at the figure.
discussed (and observed—see following) whole RC is deeply modulated. This circumstance probably allows extracting sample scattering from the result of measurements of RC for the system: ultrasonically excited DCD or BHD and sample. Above discussed calculations were done for the case of the coherent (one mode) ultrasonic wave. However in our case the value of ultrasonic wave length lSB30–50 mm, Si plate thickness B5 mm. Therefore it is difficult to excite one-mode coherent ultrasonic field. What will be happened for the case of the incoherent (multimode) ultrasonic field? The similar problem was analyzed many years before by Abragam [5] for the case of Mossbauer effect with ultrasonically excited Fe57 source. Abragam considered completely chaotic ultrasonic field model with ultrasonic amplitude Gaussian distribution function f ðwÞ ¼ 2 expðw2 =w20 Þw=w20 ; Z N w20 ¼ w2 f ðwÞw2 dw
ð4Þ
0
and successfully interpreted experimental results. We applied Abragam model for the analysis of our Neutron Spin Echo (NSE) experimental data [6]. We observed inelastic neutron scattering by ultrasound and found that NSE signal S(t) is non-negative even for the case of strong excitation. S(t) signal should be negative for the case of strong coherent ultrasonic wave. We calculated S(t) for
¼ exp ððHW0 Þ2 =2ÞIm ððHW0 Þ2 =2Þ ð5Þ where Im is the Bessel function of the second order. Therefore it is not necessary to create one-mode ultrasonic field in the DCD or BHD; we have a chance to avoid these difficult problems solution and upgrade diffractometer parameters even for the case of incoherent ultrasonic waves. We received some experimental data concerning research of inelastic neutron scattering by ultrasound by means of DC and BH diffractometers (instrument V12a, BENSC, HMI, Berlin). We used cold, slow neutrons with wavelength 0.523 nm and studied symmetrical Si(1 1 1) Bragg reflection. Longitudinal (LAW) and transversal (TAW) ultrasonic waves with frequency 70.27 MHz were excited by means of LiNbO3 transducers in perfect silicon s.c. plates. In the first series of measurements LAW was excited in Si(1 1 1) plate. Neutrons diffracted at plate were analyzed by means of Si(1 1 1) plate (DCD regime) or triple bounces Si(1 1 1) BHD analyzer (BHD regime). Our calculation predicted that BH analyzer should lead to the strong improvement of contrast with moderate loss of intensity of the main peak (Fig. 2). It happened due to the strong suppression of reflectivity outside of the Darwin plateau in the BH analyzer. The shape of the observed RC (Fig. 3) confirmed this prediction. BH analyzer ‘‘allows’’ to observe higher order ultrasonic phonon satellites. We found some results concerning DCD with simultaneously ultrasonically excited monochromator and analyser. The last term of dq (1) could be missed due to the application of cold neutron
ARTICLE IN PRESS E. Iolin et al. / Nuclear Instruments and Methods in Physics Research A 529 (2004) 152–156
2.5
155
80000
BH-analyz. DC
2
sound in analyz. two sounds
70000
1.5
Count
Intensity
60000
1
50000 40000 30000
0.5
20000 10000
0 -10
-5
0 Y
5
10
0 -50
-40
-30
-20
-10
0
10
20
30
40
50
Ang. sec.
Fig. 2. RCs calculations. Neutron scattering by ultrasonically excites silicon plate for the case of triple-bounces Bonse–Hart (red line) and DC monochromator. HW=1. Only one-phonon satellites are taken into account.
Fig. 4. DCD RCs. Monochromator: black—ultrasound switch off; red—LAW, 70.27 MHz switch on. TAW, 70.27 MHz is excited in analyzer in all cases.
3. Summary 50000
BH analyz. DC
Count
40000
30000
20000
10000
0 -50
-40
-30
-20
-10
0
10
20
30
40
50
Ang. sec.
Fig. 3. The observed RCs for the case of DC (black) and BH (red, in electronic version of paper) analyzer. Monochromator—Si plate excited by LAW, 70.27 MHz. Calculated (1) first satellite position is equal to 10.9 ang. sec.
and effects of the TAW and LAW are similar. We excited LAW in monochromator and TAW in analyzer and used two different generators with almost the same generated signal frequency, 70.27 MHz. Therefore corresponding ultrasonic excitations were not coherent with each other. It can be seen from Fig. 4 that peak intensity is seriously, B60%, increasing and simultaneously the ultrasonic phonon satellite contrast is strongly improved in this ‘‘double-sound’’ case.
We suppose to excite simultaneously high frequency ultrasound with the same frequency and non-controlled phase in DCD and BHD monochromator and analyzer and to increase intensity without loss (and even with an improvement) of resolution. This approach is confirmed by the result of corresponding calculations. It is not necessary to excite coherent (one-mode) ultrasonic wave in such measurements; incoherent ultrasonic field is applicable. We received first positive experimental results concerning ultrasonic phonon satellites research: (a) BH analyzer allows successfully observe these satellites; BH analyzer leads to the strong improvement of contrast with moderate, B20%, loss of intensity of the main peak. (b) We found some results concerning DC diffractometer with simultaneously ultrasonically excited monochromator (LAW) and analyzer (TAW). We observed that peak intensity increasing up to 60% and simultaneously ultrasonic phonon satellites contrast was strongly improved in this ‘‘double-sound’’ case. Application of cold neutrons allowed minimising some problems connected with small deformation created in perfect silicon crystal by cooplant between ultrasonic transducers and s.c.
ARTICLE IN PRESS 156
E. Iolin et al. / Nuclear Instruments and Methods in Physics Research A 529 (2004) 152–156
At our level of our knowledge both above described results were observed for the first time. It seems that ultrasonic method can be applied for the neutron scattering research in the limit of USANS and probably for improvement of the neutron imaging for the case of the refraction contrast. The observed RC (Fig. 4) is deeply modulated. This circumstance probably allows extracting sample scattering from the result of USANS measurements at the BHD with ultrasonically excited monochromator and analyzer. It seems interesting application of ultrasound for the case of the Spallation Neutron Source (SNS), generating short-wave neutrons. BH Timeof-Flight diffractometer manufactured from Si(1 1 1) plates could use simultaneously neutron harmonics with wavelength l, l/3, l/5. The same ultrasonic wave will be more effective for the case of short wave neutrons. The number of ultrasonic phonon satellites HW(l/5)=5HW(l), HW(l/3)=3HW(l). Therefore multi-phonon scat-
tering will be easily excited for the case of short wave neutrons and could be observed by means of TOF technique. At last time—amplitude modulation of the ultrasonic wave allows separately excited ultrasonic effects for neutrons with wavelength l, l/3, l/5.
References [1] E.M. Iolin, Piz’ma Zh. Techn. Fiz. 15 (1989) 52 (in Russian). [2] E.M. Iolin, L.L. Rusevich, M. Vrana, P. Mikula, P. Lukas, Phys. Status Solidi B 195 (1996) 21. [3] E. Iolin, L. Rusevich, M. Strobl, W. Treimer, P. Mikula, BENSC Experimental Report, Proposal PHY-04-884, 19/ 12/2003. [4] E. Iolin, B. Farago, F. Mezei, E. Raitman, L. Rusevich, SPIE 3239 (1997) 406. [5] A. Abragam, C. R. Acad. Sci. (France) 250 (1960) 4334. [6] E. Iolin, B. Farago, F. Mezei, E. Raitman, L. Rusevich, Physica B 241–243 (1998) 1213.