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 |
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| Дата: | 2013 |
| Автори: | , , , |
| Формат: | Стаття |
| Мова: | Англійська |
| Опубліковано: |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2013
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| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| 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. |
Репозитарії
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 .
|
| first_indexed | 2025-12-02T09:45:15Z |
| format | Article |
| 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.
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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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