Triggering of ¹⁷⁸m²Hf isomer embedded in Ta matrix by 30 keV electrons

¹⁷⁸m²Hf isomer triggering was studied using the upgraded experimental setup developed in Kharkiv National University and installed at Kyiv Institute for Nuclear Research. The target presenting a single Ta foil of 300 μm thickness with ¹⁷⁸m²Hf isomeric activity of about 100 Bq was irradiated by 30 ke...

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Опубліковано в: :Вопросы атомной науки и техники
Дата:2013
Автори: Dovbnya, A.N., Kandybei, S.S., Kirischuk, V.I., Ranyuk, Yu.N.
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Мова:Англійська
Опубліковано: Національний науковий центр «Харківський фізико-технічний інститут» НАН України 2013
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Цитувати:Triggering of ¹⁷⁸m²Hf isomer embedded in Ta matrix by 30 keV electrons / A.N. Dovbnya, S.S. Kandybei, V.I. Kirischuk, Yu.N. Ranyuk // Вопросы атомной науки и техники. — 2013. — № 3. — С. 179-186. — Бібліогр.: 32 назв. — англ.

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Digital Library of Periodicals of National Academy of Sciences of Ukraine
_version_ 1859783972479827969
author Dovbnya, A.N.
Kandybei, S.S.
Kirischuk, V.I.
Ranyuk, Yu.N.
author_facet Dovbnya, A.N.
Kandybei, S.S.
Kirischuk, V.I.
Ranyuk, Yu.N.
citation_txt Triggering of ¹⁷⁸m²Hf isomer embedded in Ta matrix by 30 keV electrons / A.N. Dovbnya, S.S. Kandybei, V.I. Kirischuk, Yu.N. Ranyuk // Вопросы атомной науки и техники. — 2013. — № 3. — С. 179-186. — Бібліогр.: 32 назв. — англ.
collection DSpace DC
container_title Вопросы атомной науки и техники
description ¹⁷⁸m²Hf isomer triggering was studied using the upgraded experimental setup developed in Kharkiv National University and installed at Kyiv Institute for Nuclear Research. The target presenting a single Ta foil of 300 μm thickness with ¹⁷⁸m²Hf isomeric activity of about 100 Bq was irradiated by 30 keV electron beam. The enhanced counting rates of all the ground-state band transitions were observed. Our data are consistent with an estimate for the triggering effect of (2.9 ± 0.7)% and corresponding triggering cross-section can qualitatively be estimated as σtrig = 4.2 × 10-²⁷ cm². Використовуючи вдосконалену експериментальну установку, розроблену в Харкiвському нацiональному унiверситетi та зiбрану в Київському Iнститутi ядерних дослiджень, вивчався трiггерiнг iзомеру ¹⁷⁸m²Hf В якостi мiшенi використовувалась танталова фольга товщиною 300мкм з ¹⁷⁸m²Hf активнiстю близько 100 Бк, яка опромiнювалась пучком електронiв з енергiєю 30 кеВ. Спостерiгалося збiльшення iнтенсивностi всiх переходiв основної смуги. За отриманими даними ефект трiггерiнга становить (2.9 ± 0.7)% i якiсна оцiнка перетину трiггерiнга вiдповiдає σtrig = 4.2 × 10−²⁷ cm². Используя усовершенствованную экспериментальную установку, разработанную в Харьковском национальном университете и собраную в Киевском Институте ядерных исследований, изучался триггеринг изомера ¹⁷⁸m²Hf. В качестве мишени использовалась танталовая фольга толщиной 300 мкм с ¹⁷⁸m²Hfактивностью около 100 Бк, которая облучалась электронным пучком с энергией 30 кэВ. Наблюдалось увеличение интенсивности всех переходов основной полосы. По полученным данным эффект триггеринга составляет (2.9 ± 0.7)% и качественная оценка сечения триггеринга соответсвует σtrig = 4.2 × 10-²⁷ cм² .
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fulltext TRIGGERING OF 178m2HF ISOMER EMBEDDED IN Ta MATRIX BY 30 keV ELECTRONS A.N. Dovbnya1, S.S. Kandybei1, V.I. Kirischuk2 ∗, Yu.N. Ranyuk1 1National Science Center ”Kharkov Institute of Physics and Technology”, Kharkov, Ukraine; 2Institute for Nuclear Research, Kiev, Ukraine (Received February 5, 2013) 178m2Hf isomer triggering was studied using the upgraded experimental setup developed in Kharkiv National Uni- versity and installed at Kyiv Institute for Nuclear Research. The target presenting a single Ta foil of 300 µm thickness with 178m2Hf isomeric activity of about 100 Bq was irradiated by 30 keV electron beam. The enhanced counting rates of all the ground-state band transitions were observed. Our data are consistent with an estimate for the triggering effect of (2.9 ± 0.7)% and corresponding triggering cross-section can qualitatively be estimated as σtrig = 4.2× 10−27 cm2. PACS: 23.20.Nx; 25.20.Dc; 27.70.+q 1. BACKGROUND AND INTRODUCTION The 16+ 4-quasiparticle state of the nucleus 178Hf, a K isomer with the excitation energy Ee = 2.4474 MeV and half-life T1/2 = 31 years, is considered as the most promising on the way to create gamma-ray sources controlled by low energy photons and a gamma ray laser as well. Having so high excita- tion energy and the longest half-life among all known highly excited nuclear isomers, 178m2Hf isomer is ab- solutely unique nuclear isomer and the most perspec- tive for the triggering experiments. Consequently, this isomer has become the subject of intense ex- perimental study for possible mechanisms that could trigger its decay. Since 1998 a number of experiments have been performed, nevertheless the obtained posi- tive results ([1]-[10]) completely exclude the negative results ([11]-[13]) and vice versa yet. The main attention of all recently conducted ex- periments has been focused upon 178m2Hf isomer since, first, its high excitation energy allows to pro- pose a great variety of possible triggering mechanisms and, secondly, its longest half-life ensures the high- est sensitivity of the triggering experiments. Such uniqueness of 178m2Hf isomer creates an essential challenge for the triggering experiments as well. On the one hand, the production of lab-sized quantities (∼ 103...105 Bq) happens to be very difficult task, on the other hand, the more productive nuclear reac- tion is chosen, the longer cooling time (usually from 6 to 20 years) is required for the acceptable reduction of by-products activity. As a result, only one ex- perimental group, though using very similar 178m2Hf isomer target of the same origin (Los Alamos high current accelerator) and the same irradiation facility (SPring-8 Synchrotron), has really repeated the trig- gering experiments claiming the positive results, how- ever failed to confirm the triggering [13]. Additional efforts of the joint Lawrence Livermore, Los Alamos and Argonne team to check the principal possibility of 178m2Hf isomer triggering ([11]-[12]), just as the San- dia team to repeat the triggering experiment at the National Synchrotron Light Source of Brookhaven National Laboratory in 2005 (so called TRiggered Isomer Proof Test though confirming the triggering of 178m2Hf isomer, but its results have never been pub- lished yet) have not clarified completely the situation. At the same time, taking into account the practically absolute inaccessibility of 178m2Hf isomer targets and significant experimental difficulties, it is not easy to apply the new efforts to the detailed study of 178m2Hf isomer triggering. The idea of 178m2Hf isomer triggering experi- ments is very simple and clever. The depopula- tion of 180mTa isomer ([14]-[15]) and excitation of 123mTe, 125mTe [16] and other long-lived isomers [17] with high-energy bremsstrahlung have shown that the energy stored in isomeric levels can be triggered by photons. All these experiments have reliably demonstrated that the intermediate states through which the triggering occurs can be found at the excitation energies about 2.8 MeV. It has been quite straightforward to suppose that if in- stead of the ground or low excited state of a nu- cleus some highly excited nuclear state is used then the analogous triggering effect could be reached us- ing photons with much lower energy. Thus, pho- tons with the energy of around 300 keV can be enough to observe 178m2Hf isomer triggering. And although nuclear theory predicts lots of such highly excited isomeric states, along with 178m2Hf only two ∗Corresponding author E-mail address: kirisch@mpca.kiev.ua ISSN 1562-6016. PROBLEMS OF ATOMIC SCIENCE AND TECHNOLOGY, 2013, N3(85). Series: Nuclear Physics Investigations (60), p.179-186. 179 of them were discovered till now that are long-lived 177mLu (Ee = 970.2 keV, T1/2 = 160.5 d) and 179mHf (Ee = 1.1057 MeV, T1/2 = 25.1 d). Further experi- ments performed with the enriched 180mTa targets and more intense bremsstrahlung sources have clearly shown that the triggering of 180mTa isomer takes place, though with more than 3 orders of magnitude less yield, even at the excitation energies a little above 1 MeV [18], hence all mentioned above long-lived and highly excited isomers can be considered as very at- tractive for the triggering experiments. The half-life and excitation energy are extremely crucial parameters for the sensitivity of triggering ex- periments. First, for approximately the same trig- gering cross-sections, while the triggering effect used to be detected as the extra counts over the spon- taneous decays of the isomer, the longer half-life of the isomer the more sensitive triggering experiment. Only due to the differences in half-lives 178m2Hf trig- gering experiments are about 60 and 450-fold more sensitive, than 177mLu and 179mHf triggering experi- ments, respectively. Secondly, the density of nuclear levels increases significantly at higher energies, thus the higher excitation energy of a nuclear isomer the higher probability to exist for any K-mixing nuclear states through which the triggering could proceed. As a result, 178m2Hf isomer having so high excitation energy and the longest half-life is absolutely unique nuclear isomer and the most perspective for the trig- gering experiments. 2. 178m2Hf ISOMER TRIGGERING MECHANISMS, PRODUCTION AND AVAILABLE SOURCES The main problem of 178m2Hf isomer triggering is that any available theoretical calculations cannot pre- dict such high cross-section for the experimentally ob- served effect and the discrepancy is the orders of mag- nitude. From all the proposed mechanisms including the classical Nuclear Excitation by Electron Transi- tion (NEET) ([7]-[8]), electronic bridges (both elas- tic and non-elastic ones) [19], resonant internal con- version [20] and NEET through autoionization states [21] that could explain the results of 178m2Hf isomer triggering experiments the classical NEET is consid- ered as the most probable. The NEET effect has been studied in detail in 197Au for which the energy misbalance between cor- responding nuclear and atomic transitions was con- sidered to be as small as 51 ± 2 eV. The new exper- iment on NEET observation in 197Au conducted re- cently at SPring-8 [22] has clearly indicated that the energy misbalance is 40±2 eV and only recently this disagreement has experimentally been resolved [23]. The last experiment on NEET observation in 197Au and following theoretical calculations [24] have revealed some NEET features registered in 178m2Hf isomer triggering experiments as well. 178m2Hf iso- mer triggering experiments conducted in 2002 and 2003 at SPring-8 have clearly demonstrated that the triggering effect is recorded when the photon ener- gies are a little higher than corresponding L edges [7]. The most reliable triggering effect in 178m2Hf isomer has been observed in the experiments when the pho- ton energy is around 6 eV above L3 edge [8], thus it could mean that an energy misbalance for the corresponding nuclear transition (between the iso- meric and intermediate levels) and atomic transition (Mx ⇒ L3) in 178Hf is about 6 eV. In both NEET cases the mentioned above energy misbalances hap- pen to be close to the natural widths of correspond- ing atomic levels WK(Au) = 52 eV and WL3(Hf) = 4.9 eV. And though it could be quite natural that NEET width should depend on the widths of the fi- nal and initial states (beam width is much narrower while the nuclear level width is negligibly small com- pared to the atomic shell widths), the last NEET ex- periment in 197Au has evidently demonstrated that WNEET (197Au) = 14±9 eV, thus K-shell width does not contribute to the NEET effect. At the same time, in 178m2Hf isomer triggering experiments conducted in 2003 it has been observed that WNEET (178m2Hf) is about 1 eV, i.e. less than L3-subshell width in Hf as well. The above-mentioned uniqueness of 178m2Hf iso- mer has forced the intense search all over the world for any Ta targets irradiated by high energy pro- jectiles many years ago, just as in the case of high current accelerator LAMPF/LANSCE at LANL [25], leading to the discovery of Ta samples irradiated by 4.5 and 1.2 GeV electrons at Erevan Synchrotron [26] and Kharkov Linac [27], respectively. A set of Ta foils with 178m2Hf isomer activity has recently been found at Kiev Institute for Nuclear Re- search as well. All these foils of 100 µm thickness were used as the partial energy absorbers many years ago in nuclear experiments with 100 MeV α-particles. At such energies of α-particles the original contaminant production is not very high and the most undesirable by-product 172Hf, presented in significant quantities in all 178m2Hf isomeric targets available yet, has not been produced at all. Now the most part of con- taminants decayed completely and the total activity of the foils is presented only by 178m2Hf and 179Ta (T1/2 = 664.5 d) [28]. 179Ta decays exclusively by electron capture to 179Hf ground state and there are no γ-rays in its spectrum that can be recorded by HPGe detector save for Hf characteristic x-rays [29]. Thus 179Ta, while it can be removed by subsequent chemical processing and extraction of Hf fraction, is a minor problem for 178m2Hf triggering experiments. 3. TARGET, EXPERIMENTAL SETUP AND RESULTS Taking into account all the experience acquired dur- ing the initial 178m2Hf triggering experiments with dental x-ray machine, a new experimental setup has been developed at Kharkov National University and installed at Kiev Institute for Nuclear Research al- lowing the irradiation of isomeric targets directly by electrons (or by x-rays when corresponding converters 180 are used) with the energy of 1...25 keV and currents 0-150 µA. Two modifications of experimental setup, one for single γ-ray spectra measurements and an- other for coincidence γ-ray spectra measurements in very close geometry (as close as possible to 2π and 4π geometry, correspondingly) have been constructed. Low energy bremsstrahlung radiation has essen- tially been absorbed by the experimental setup wall to quite acceptable levels without noticeable decrease of 178m2Hf γ-rays, even the lowest energy ones. Addi- tionally, the target unit of the experimental setup has been heavily and rather effectively shielded against natural γ-ray background by more than 10 cm of Pb. Therefore, according to our estimates the sensitivity of 178m2Hf triggering experiments must be orders of magnitude higher than in the case of the initial ex- periments with dental x-ray machine. The given 178m2Hf isomer triggering experiment has been conducted using as a target the Ta foil of 300 µm thickness with about 100 Bq isomeric activ- ity which was exploited many years ago as the con- verter at Kharkov 1.2 GeV Linac [27]. Such target has at least two-fold advantage. First, it can be irra- diated directly by electrons at maximal currents with- out any risk to be overheated and evaporated into vacuum. Secondly, in order to find any reasonable explanation for unexpectedly high triggering cross- section obtained in the initial 178m2Hf triggering ex- periments it has been proposed to use the pump- ing radiation derived from x-ray line spectrum of a medium chosen so that one of its strong x-ray lines is resonant with the nuclear transition to the inter- mediate state [30]. Thus, our experiments have been performed with 178m2Hf target when for the maxi- mal efficiency of characteristic x-rays the isomer is embedded in Ta matrix. Taking into account that the used isomeric target is rather thick, the experimental setup has been upgraded to the energy 1...30 keV and currents 0...250 µA and the triggering experiment has been carried out with 30 keV electron beam at the average current higher than 200 µA. The diameter of the beam spot at the target was about 8 mm – around the same size as the areas of two available spots each with practically the same 178m2Hf isomer activity in the target. The γ-ray spectra have been acquired using 18% HPGe coaxial detector mounted on the opposite side of the target in the horizontal plane and at 180◦ to the horizontally incident electron beam. In our experiment GC 2018 (CANBERRA) detector with the efficiency 20% and energy resolu- tion at γ1332 keV peak of 60Co better than 1.8 keV and the standard acquisition system based on CAN- BERRA InSpector 2000 unit have been used. The distance from the detector front face to the target taking into account the thickness of the experimental setup wall was less than 5 mm. The acquisition rate in γ213 keV peak of 178m2Hf isomer (Fig. 1) even tak- ing into consideration its essential absorption in Ta and additional absorption in the experimental setup wall and the detector cap was around 4.3 counts/s. Fig.1. Energy level diagram illustrating the sponta- neous and induced decay of the 31-yr isomer of 178Hf Lx lines of Hf cannot be detected in our experiment, consequently the bremsstruhlung radiation with the endpoint energy of 30 keV has been used as the beam- on monitor of electron currents at the target in all runs of measurements ensuring that the experimental luminosity remain at the expected values during the irradiations (Fig. 2,a). At the same time, bremsstruh- lung radiation rate registered by the detector was low enough for x-ray coincidence detection (Fig. 2,b). Fig.2. Bremsstruhlung radiation with the endpoint energy of 30 keV used as the beam-on monitor of electron currents at the target in all runs of the irradiations. (a) Due to the essential absorption of low-energy X-rays the bremsstruhlung radiation is registered by HPGe detector as a peak with the maximum at the energy about 7 keV lower than the endpoint energy. (b) Bremsstruhlung radiation rate registered by the detector was too low to produce the summing peak The entire series of measurements have involved a number of runs of beam-on measurements with the absolutely stable measurement geometry and the 181 total acquisition period of about 8 hours. In or- der to monitor the stability of measurement geom- etry, several runs of beam-off measurements have been conducted before and after the irradiations, just as between the separate beam-on measure- ments each accumulating data over the periods from a few hours to around 20 hours. Table 1 shows the counting rates of all GSB transitions acquired in the beam-off runs conducted before and after the irradiations used to obtain the av- eraged values for above-mentioned counting rates. Table 1. The counting rates of all GSB transitions obtained in the beam-off runs conducted before and after the irradiations Transition Intensity Intensity Averaged before the after the intensity irradiation irradiation γ93 keV 0.208(2) 0.215(2) 0.212(4) γ213 keV 4.27(2) 4.28(3) 4.28(2) γ325 keV 3.77(1) 3.79(1) 3.78(1) γ426 keV 3.01(1) 3.03(1) 3.02(1) The accuracies of obtained counting rates are bet- ter than 1% even in the case of γ93 keV transition having the lowest intensity and reaches 0.3% for the transitions with the highest intensities. In the event when the statistical uncertainty of averaged intensity turned out to be less than the data spread obtained before and after the irradiations the latter has been taken as the uncertainty for the corresponding aver- aged intensity. The beam-induced decay of the isomer has re- sulted in the increase of the GSB transition inten- sities compared to the beam-off measurements (Ta- ble 2). While any new γ-ray peaks not observed in the spontaneous decay of 178m2Hf isomer have not been detected in this experiment, such result allows to use the total counting rate of mentioned above cascade transitions as a measure of the triggering effect. The total counting rates of all transitions triggered in the decay of 178m2Hf isomer have been registered at the levels 11.451 ± 0.032 and 11.285 ± 0.021 decays per second for the beam-on and beam-off spectra, respec- tively, thus the enhancement factor can be estimated as (1.47 ± 0.34)%. Since there are two spots with practically the same 178m2Hf isomer activity in the target and only one of them has been irradiated in the given experiment, the real relative triggering ef- fect turns out to be two fold higher (2.9± 0.7)%. In particular, this has resulted in the enhanced counting rates of all GSB transitions compared to the counting rates of the γ88 keV isomeric and 8−-state band transitions as well. Table 3 shows the counting rates of the γ88 keV isomeric and all 8− band transi- tions (save for γ277 keV transition since its intensity is about order of magnitude lower than the inten- sity of the weakest transition in the list) acquired in the beam-off runs conducted before and after the irradiations used to obtain the averaged values for above-mentioned counting rates. The accuracies of obtained counting rates are better than 1% even in the case of γ534 keV transition having the lowest intensity and reaches 0.25% for the transitions with the highest intensities. In the event when the sta- tistical uncertainty of averaged intensity turned out to be less than the data spread obtained before and after the irradiations the latter has been taken as the uncertainty for the corresponding averaged intensity. Table 2. The enhanced beam-on counting rates of all GSB transitions compared to the averaged beam-off counting rates Transition Beam-on Beam-off Enhancement intensity intensity γ93 keV 0.215(5) 0.212(4) +0.003(6) γ213 keV 0.215(5) 4.28(2) +0.05(3) γ325 keV 3.84(2) 3.78(1) +0.06(2) γ426 keV 3.07(1) 3.02(1) +0.05(2) Total GSB transition 11.45(3) 11.29(2) +0.17(4) Relative enhancement +1.47(34)% Table 3. The counting rates of the γ88 keV isomeric and 8− band transitions obtained in the beam-off runs conducted before and after the irradiations Transition Intensity Intensity Averaged before the after the intensity irradiation irradiation γ88 keV 0.674(3) 0.679(4) 0.676(3) γ216 keV 3.52(2) 3.55(2) 3.53(2) γ237 keV 0.433(3) 0.423(4) 0.429(5) γ257 keV 0.791(5) 0.803(4) 0.797(6) γ297 keV 0.417(5) 0.416(5) 0.416(4) γ454 keV 0.565(3) 0.571(4) 0.567(3) γ495 keV 2.034(8) 2.034(7) 2.034(5) γ534 keV 0.253(2) 0.250(3) 0.252(2) γ574 keV 2.114(6) 2.131(7) 2.122(8) The beam-induced decay of the isomer has resulted in the non-enhanced beam-on counting rates of the γ88 keV isomeric and 8− band transitions compared to the averaged beam-off counting rates (see Table 4) and the non-enhancement factor can be estimated as (0.0221± 0.0366)%. The relevant portion of difference spectrum for the total beam-on and beam-off spectra normalized to the same period of time is presented in Fig. 3. The channel widths are ∼ 0.152 keV/channel. Similar difference spectra for different beam-off and beam- on spectra normalized to the same period of time are presented in Fig. 4 and Fig. 5, respectively. The reference beam-off spectrum shown above the acquired difference spectra presents the difference spectrum between the beam-on and beam-off spec- tra measured in the ideal case when no calibra- tion shifts and line width broadenings are observed 182 while the triggering effect is exactly equal to 1.47%. Table 4. The non-enhanced beam-on counting rates of the γ88 keV isomeric and 8− band transitions compared to the averaged beam-off counting rates Transition Beam-on Beam-off Enhancement intensity intensity γ88 keV 0.673(6) 0.676(3) -0.003(7) γ216 keV 3.53(3) 3.53(3) -0.005(29) γ237 keV 0.421(9) 0.429(5) -0.01(1) γ257 keV 0.790(6) 0.797(6) -0.007(9) γ297 keV 0.427(5) 0.416(4) +0.011(6) γ454 keV 0.565(5) 0.567(3) -0.002(6) γ495 keV 2.038(9) 2.034(5) +0.004(10) γ534 keV 0.256(4) 0.252(2) +0.004(4) γ574 keV 2.15(1) 2.122(5) +0.028(11) Averaged enhancement +0.022(37) Fig.3. The points with error bars show the differ- ence gamma-ray spectrum between the beam-on and beam-off spectra from 205 to 225 keV normalized to the same period of time Fig.4. The points with error bars show the differ- ence gamma-ray spectrum between different beam-off spectra from 205 to 225 keV normalized to the same period of time In our experiment the line widths have significantly broadened in the beam-on spectra compared to the beam-off spectra. Such broadening has pro- duced very strong differentiation effect in the differ- ence spectrum not allowing to illustrate visually the 178m2Hf isomer triggering in the difference spectrum as it used to be done earlier. All the reasons for the differentiation are quite clearly shown in Fig. 6. Nevertheless, in the case of the difference spec- trum for different beam-on spectra (see Fig. 5), when the relative line width broadening is not so drastic as in the event of the difference spectrum for beam-on and beam-off spectra (see Fig. 3), a slight triggering effect due to some difference in beam currents is rather visible for γ213 keV peak. Fig.5. The points with error bars show the differ- ence gamma-ray spectrum between different beam-on spectra from 205 to 225 keV normalized to the same period of time when irradiation currents differ slightly Fig.6. Notably broadened line widths observed in the beam-on spectra compared to the beam-off spectra are a reason why the very strong differentiation is clearly seen in the difference spectrum 4. DISCUSSIONS AND CONCLUSIONS In the same manner to that used in the previous works we can estimate, at least qualitatively and ig- noring many unknown effects including all the ab- sorption factors, the triggering cross-section as well. The enhancement factor S can be expressed through the triggering cross-section from the relation S · (N/τ) = (N/A) · F · σtrig, (1) 183 where N is the number of isomeric states in the target, τ is the lifetime of the isomeric state (1.4× 109 s), A is the area of the target, F is the number of incident electrons, and σtrig is the triggering cross-section for the isomer de-excitation. N/τ is the normal decay rate of the isomeric nuclei in the target. This then gives σtrig = S · [A/(τ × F )] . (2) As a result, for the observed triggering effect S = 2.9% with the values of A = 0.8 cm2 and F = 4× 1015 e/s (∼ 200 µA), the cross-section esti- mate σtrig = 4.2× 10−27 cm2 can be deduced. The most part of publications in which the prin- cipal possibility of 178m2Hf isomer triggering is ques- tioned used to be based on the contrary proposi- tions – if the size of detected triggering effect is correct or some feature of the triggering process is really recorded, then the corresponding triggering cross-sections (for γ-ray absorption, etc.) and prob- abilities (for NEET effect, high multipolarity tran- sitions, etc.) turn out to be orders of magnitude higher than one could expect from the available sys- tematics. Now that is hardly the case. In the last published work on 178m2Hf isomer triggering [13] the authors has obtained the upper limit for the mag- nitude of the triggering effect cross-section around 7 × 10−27 cm2 keV, which is quite comparable with the upper limit from the null experiment [12] of about 3× 10−27 cm2 keV. If the real photon flux of around 3.3× 1012 photons/cm2/s used in the experiment is taken into account instead of the spectral flux den- sity applied to produce the above-mentioned esti- mate, one can obtain the upper limit for the cross- section magnitude of the triggering effect expressed in cm2 (or barns) and it turns out to be about 4.6 × 10−24 cm2 (or 4.6 barns). It means that in our case if all the losses of electron beam and pro- duced bremsstrahlung radiation (including the shield- ing of 178m2Hf isomer by Ta matrix, ineffectiveness of the wide-range bremsstrahlung radiation compared to the practically monochromatic photon beam at the SPring-8 beamline, etc.) and possible multiplication effects (for instant, when a single electron with the energy ≤ 30 keV can ionize more then a single atomic shell in Hf, etc.) are taken into account, then the av- eraged effectiveness of electron flux happens to be at the level of around 10−3 and it sounds more or less realistic. And while the branching ratios or partial prob- abilities for an assumed intermediate state to decay back to the isomeric level [31] can really be very high when one supposes that to bypass the isomeric level the intermediate state should decay through any well- known excited level of 178Hf, for instant 13− level of 8− band of 178Hf [31], such particular scheme for the induced decay of 178m2Hf isomer can have noth- ing to do with the real situation. For example, the intermediate state should not necessarily be a band head state and in this case its decay by an intraband transition, the probability of which used to be higher than the probability of crossover (or interband) tran- sitions, is quite possible. On the other hand, the intermediate state can be so-called γ-soft one pre- senting the mixture of practically all K-values and for such excited level the decay probabilities to any nuclear levels with the same nuclear spin and par- ity could have rather comparable magnitudes. More- over, the current theoretical study of possible NEET effect for 178m2Hf isomer in the frame of strict colli- sion theory [32] indicates that the above-mentioned controversy is not as drastic as it has been considered before as well ([13], [31]). In summary, we have repeated the initial 178m2Hf isomer triggering experiments using the isomeric source not used before and the new experimental setup upgraded for this experiment. We see the evi- dence for the triggering of 178m2Hf isomer by observ- ing the enhanced counting rates of all ground-state band transitions. Our data are consistent with an estimate for the triggering effect of (2.9± 0.7)%. Additionally, it has been demonstrated that even using rather weak isomeric source the sensitivity of our 178m2Hf isomer triggering experiments is much better than in the initial triggering experiments with dental x-ray machine. More detailed conclusions can be made when much stronger isomeric source will be prepared and ready for use. In this case the increased emission of γ-rays could be detected in every separate transi- tion supplying exclusively valuable information about possible scenarios and mechanisms of the induced ac- celeration of 178m2Hf isomer decay. It would allow to conduct the coincidence measurements as well. ACKNOWLEDGEMENTS We are indebted to Prof. Valentine V. Chorny and his colleagues for their help in the development of new experimental setup and to Prof. Aleksey I. Feoktis- tov and Dr. Vladimir T. Kupryashkin for their help in various aspects of these measurements. Special thanks to Dr. Nikolay V. Strilchuk for the encour- agement of this investigation. References 1. C.B.Collins, F.Davanloo, M.C. Iosif, et al. Evi- dence for the Forced Gamma Emission from the 31-Year Isomer of Hafnium-178 // Laser Phys. 1999, v. 9, p. 8-11. 2. C.B.Collins, F. Davanloo, M.C. Iosif, et al. Accel- erated Emission of Gamma Rays from the 31-yr Isomer of 178Hf Induced by X-Ray Irradiation // Phys. Rev. Lett. 1999, v. 82, p. 695-698. 3. C.B. Collins, F. Davanloo, N.C. Zoita, et al. γ emission from the 31-yr isomer of 178Hf induced by x-ray irradiation // Phys. Rev. C. 2000, v. 61, p. 054305. 184 4. C.B. Collins, F. Davanloo, M.C. Iosif, et al. Study of the gamma emission from the 31-yr isomer of 178Hf induced by x-ray irradiation // Phys. of Atomic Nuclei. 2000, v. 63, p. 2067-2072. 5. C.B. Collins, A.C. Rusu, N.C. Zoita, et al. Gamma-Ray Transitions Induced in Nuclear Spin Isomers by X-Rays // Hyperfine Interactions. 2001, v. 135, p. 51-70. 6. C.B. Collins, N.C. Zoita, A.C. Rusu, et al. Scal- ing of Gamma-Ray Transitions Induced in Nu- clear Spin Isomers by X-rays // Laser Phys. 2001, v. 11, p. 1-5. 7. C.B. Collins, N.C. Zoita, A.C. Rusu, et al. Tun- able synchrotron radiation used to induce γ- emission from the 31 year isomer of 178Hf // Eu- rophys. Lett. 2002, v. 57, p. 677-682. 8. C.B. Collins, N.C. Zoita, F .Davanloo, et al. Ac- celerated γ-emission from isomeric nuclei // Rad. Phys. and Chem. 2004, v. 71, p. 619-625. 9. C.B. Collins, N.C. Zoita, F. Davanloo, et al. Ac- celerated Decay of the 31-yr Isomer of Hf-178 In- duced by Low-Energy Photons and Electrons // Laser Phys. 2004, v. 14, p. 154-165. 10. V.I. Kirischuk, P. McDaniel, C.B. Collins, et al. // The Proceedings of the 7th AFOSR Workshop ”Isomers and Quantum Nucleonics”. Dubna, Russia, 2005, p. 99. 11. I. Ahmad, J.C. Banar, J.A. Becher, et al. Search for X-Ray Induced Acceleration of the Decay of the 31-Yr Isomer of 178Hf Using Synchrotron Ra- diation // Phys. Rev. Lett. 2001, v. 87, p. 072503. 12. I. Ahmad, J.C. Banar, J.A. Becher, et al. Search for x-ray induced decay of the 31-yr isomer of 178Hf using synchrotron radiation // Phys. Rev. C. 2005, v. 71, p. 024311. 13. J.J. Carroll, S.A. Karamian, R. Propri, et al. Search for low-energy induced depletion of 178Hfm2 at the SPring-8 synchrotron // Phys. Lett. B. 2009, v. 679, p. 203-208. 14. C.B. Collins, C. D. Eberhard, J.W. Gle- sener, and J.A. Anderson. Depopulation of the isomeric state 180Tam by the reaction 180Tam(γ, γ′)180Ta // Phys. Rev. C. 1988, v. 37, p. 2267-2269. 15. C.B. Collins, J.J. Carroll, T.W. Sinor, et al. Resonant excitation of the reaction 180Tam(γ, γ′)180Ta // Phys. Rev. C. 1990, v. 42, p.R1813-R1816. 16. J.J. Carroll, T.W. Sinor, D.G. Richmond, et al. Excitation of 123Tem and 125Tem through (γ, γ′) reactions // Phys. Rev. C. 1991, v. 43, p. 897-900. 17. J.J. Carroll, M.J. Byrd, D.G. Richmond, et al. Photoexcitation of nuclear isomers by (γ, γ′) re- actions // Phys. Rev. C. 1991, v. 43, p. 1238-1247. 18. D. Belic, C. Arlandini, J. Besserer, et al. Pho- toactivation of 180Tam and Its Implications for the Nucleosynthesis of Nature’s Rarest Naturally Occurring Isotope // Phys. Rev. Lett. 1999, v. 83, p. 5242-5245. 19. V.I. Kirischuk, N.V. Strilchuk and V.A. Zheltonozhsky // The Proceedings of the II International Conference ”Frontiers of Nonlinear Physics”. Nizhni Novgorod, Russia, 2005, p. 477. 20. F.F. Karpeshin, M.B. Trzhaskovskaya, J. Zhang. Prospect of triggering the 178m2Hf isomer and the role of resonance conversion // Eur. Phys. J. A. 2009, v. 39, p. 341-348. 21. I.N. Izosimov. Triggering of Nuclear Isomers via Decay of Autoionization States in Electron Shells (NEET) // Laser Phys. 2007, v. 17, p. 755. 22. S. Kishimoto, Y. Yoda, Y. Kobayashi et al. Nu- clear excitation by electron transition on 197Au by photoionization around the K-absorption edge // Phys. Rev. C. 2006, v. 74, p. 031301(R). 23. V. Kirischuk, A. Savrasov, N. Strilchuk and V. Zheltonozhsky. Precise energy measurements of the first-excited state in 197Au // Europhys. Lett. 2012, v. 97, p. 32001. 24. A.Ya. Dzyublik. Photo-induced nuclear excita- tion by electron transition // JETP Lett. 2011, v. 93, p. 547-551. 25. S.A. Karamiana, J. Adama, D.V. Filossofov, et al. Accumulation of the 178m2Hf isomeric nuclei through spallation with intermediate-energy pro- tons of tantalum and rhenium targets // JETP Lett. 2002, v. 489, p. 448-468. 26. S.A. Karamian, J.J. Carroll, J. Adam, N.A. De- mekhina. Production of the 178m2Hf isomer us- ing a 4.5-GeV electron accelerator // JETP Lett. 2004, v. 530, p. 463-472. 27. I.G. Goncharov, A.M. Dovbnya, Yu.M. Ranyuk, et la. LONGLIVING 178m2Hf, 172Hf, 150Eu AND 133Ba ISOTOPES PHOTOPRODUC- TION STUDDING // Problems of Atomic Sci- ence and Technology 2007, v. 48, p. 22-25. 28. V.I. Kirischuk // In preparation. 29. R.B. Firestone, et al. Table of Isotopes, Eighth Edition. New York: ”John Wiley and Sons”, 1996. 30. C.B. Collins, F.W. Lee, D.M. Shemwell, et al. The coherent and incoherent pumping of a gamma ray laser with intense optical radiation // J. Appl. Phys. 1982, v. 53, p. 4645-4651. 31. M.R. Harston, J.J. Carroll. Limits on Nuclear Excitation and Deexcitation of 178Hfm2 by Elec- tron–Nucleus Coupling // Laser Phys. 2005, v. 15, p. 487-491. 32. A.Ya. Dzyublik. // In preparation. 185 ТРИГГЕРИНГ ИЗОМЕРА 178m2Hf, ВНЕДРЁННОГО В Ta-МАТРИЦУ ПРИ ОБЛУЧЕНИИ 30 кэВ ЭЛЕКТРОНАМИ А.Н. Довбня, С.С. Кандыбей, В.И. Кирищук, Ю.Н. Ранюк Используя усовершенствованную экспериментальную установку, разработанную в Харьковском наци- ональном университете и собраную в Киевском Институте ядерных исследований, изучался тригге- ринг изомера 178m2Hf. В качестве мишени использовалась танталовая фольга толщиной 300 мкм с 178m2Hf активностью около 100 Бк, которая облучалась электронным пучком с энергией 30 кэВ. На- блюдалось увеличение интенсивности всех переходов основной полосы. По полученным данным эф- фект триггеринга состовляет (2.9 ± 0.7)% и качественная оценка сечения триггеринга соответсвует σtrig = 4.2× 10−27 см2. ТРIГГЕРIНГ IЗОМЕРУ 178m2Hf, ВБУДОВАННОГО В Ta-МАТРИЦЮ ПРИ ОПРОМIНЕННI 30 кеВ ЕЛЕКТРОНАМИ А.М. Довбня, С.С. Кандибей, В.I. Кирищук, Ю.М. Ранюк Використовуючи вдосконалену експериментальну установку, розроблену в Харкiвському нацiональ- ному унiверситетi та зiбрану в Київському Iнститутi ядерних дослiджень, вивчався трiггерiнг iзомеру 178m2Hf. В якостi мiшенi використовувалась танталова фольга товщиною 300 мкм з 178m2Hf активнiстю близько 100 Бк, яка опромiнювалась пучком електронiв з енергiєю 30 кеВ. Спостерiгалося збiльшен- ня iнтенсивностi всiх переходiв основної смуги. За отриманими даними ефект трiггерiнга становить (2.9± 0.7)% i якiсна оцiнка перетину трiггерiнга вiдповiдає σtrig = 4.2× 10−27 см2. 186
id nasplib_isofts_kiev_ua-123456789-111850
institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
issn 1562-6016
language English
last_indexed 2025-12-02T09:45:15Z
publishDate 2013
publisher Національний науковий центр «Харківський фізико-технічний інститут» НАН України
record_format dspace
spelling Dovbnya, A.N.
Kandybei, S.S.
Kirischuk, V.I.
Ranyuk, Yu.N.
2017-01-15T11:37:45Z
2017-01-15T11:37:45Z
2013
Triggering of ¹⁷⁸m²Hf isomer embedded in Ta matrix by 30 keV electrons / A.N. Dovbnya, S.S. Kandybei, V.I. Kirischuk, Yu.N. Ranyuk // Вопросы атомной науки и техники. — 2013. — № 3. — С. 179-186. — Бібліогр.: 32 назв. — англ.
1562-6016
PACS: 23.20.Nx; 25.20.Dc; 27.70.+q
https://nasplib.isofts.kiev.ua/handle/123456789/111850
¹⁷⁸m²Hf isomer triggering was studied using the upgraded experimental setup developed in Kharkiv National University and installed at Kyiv Institute for Nuclear Research. The target presenting a single Ta foil of 300 μm thickness with ¹⁷⁸m²Hf isomeric activity of about 100 Bq was irradiated by 30 keV electron beam. The enhanced counting rates of all the ground-state band transitions were observed. Our data are consistent with an estimate for the triggering effect of (2.9 ± 0.7)% and corresponding triggering cross-section can qualitatively be estimated as σtrig = 4.2 × 10-²⁷ cm².
Використовуючи вдосконалену експериментальну установку, розроблену в Харкiвському нацiональному унiверситетi та зiбрану в Київському Iнститутi ядерних дослiджень, вивчався трiггерiнг iзомеру ¹⁷⁸m²Hf В якостi мiшенi використовувалась танталова фольга товщиною 300мкм з ¹⁷⁸m²Hf активнiстю близько 100 Бк, яка опромiнювалась пучком електронiв з енергiєю 30 кеВ. Спостерiгалося збiльшення iнтенсивностi всiх переходiв основної смуги. За отриманими даними ефект трiггерiнга становить (2.9 ± 0.7)% i якiсна оцiнка перетину трiггерiнга вiдповiдає σtrig = 4.2 × 10−²⁷ cm².
Используя усовершенствованную экспериментальную установку, разработанную в Харьковском национальном университете и собраную в Киевском Институте ядерных исследований, изучался триггеринг изомера ¹⁷⁸m²Hf. В качестве мишени использовалась танталовая фольга толщиной 300 мкм с ¹⁷⁸m²Hfактивностью около 100 Бк, которая облучалась электронным пучком с энергией 30 кэВ. Наблюдалось увеличение интенсивности всех переходов основной полосы. По полученным данным эффект триггеринга составляет (2.9 ± 0.7)% и качественная оценка сечения триггеринга соответсвует σtrig = 4.2 × 10-²⁷ cм² .
We are indebted to Prof. Valentine V. Chorny and his colleagues for their help in the development of new experimental setup and to Prof. Aleksey I. Feoktistov and Dr. Vladimir T. Kupryashkin for their help in various aspects of these measurements. Special thanks to Dr. Nikolay V. Strilchuk for the encouragement of this investigation.
en
Національний науковий центр «Харківський фізико-технічний інститут» НАН України
Вопросы атомной науки и техники
Ядерная физика и элементарные частицы
Triggering of ¹⁷⁸m²Hf isomer embedded in Ta matrix by 30 keV electrons
Трiггерiнг iзомеру ¹⁷⁸m²Hf, вбудованного в Ta-матрицю при опромiненнi 30 кеВ електронами
Триггеринг изомера ¹⁷⁸m²Hf, внедрённого в Ta-матрицу при облучении 30 кэВ электронами
Article
published earlier
spellingShingle Triggering of ¹⁷⁸m²Hf isomer embedded in Ta matrix by 30 keV electrons
Dovbnya, A.N.
Kandybei, S.S.
Kirischuk, V.I.
Ranyuk, Yu.N.
Ядерная физика и элементарные частицы
title Triggering of ¹⁷⁸m²Hf isomer embedded in Ta matrix by 30 keV electrons
title_alt Трiггерiнг iзомеру ¹⁷⁸m²Hf, вбудованного в Ta-матрицю при опромiненнi 30 кеВ електронами
Триггеринг изомера ¹⁷⁸m²Hf, внедрённого в Ta-матрицу при облучении 30 кэВ электронами
title_full Triggering of ¹⁷⁸m²Hf isomer embedded in Ta matrix by 30 keV electrons
title_fullStr Triggering of ¹⁷⁸m²Hf isomer embedded in Ta matrix by 30 keV electrons
title_full_unstemmed Triggering of ¹⁷⁸m²Hf isomer embedded in Ta matrix by 30 keV electrons
title_short Triggering of ¹⁷⁸m²Hf isomer embedded in Ta matrix by 30 keV electrons
title_sort triggering of ¹⁷⁸m²hf isomer embedded in ta matrix by 30 kev electrons
topic Ядерная физика и элементарные частицы
topic_facet Ядерная физика и элементарные частицы
url https://nasplib.isofts.kiev.ua/handle/123456789/111850
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