Incident X-ray Energy Research Articles

Comparative evaluation of single crystal scintillators under x-ray imaging conditions

The present study is a comparative investigation of the luminescence properties of (Lu,Y)2SiO5: Ce (LYSO: Ce), YAlO3: Ce (YAP: Ce), Gd2SiO5: Ce (GSO: Ce) and (Bi4Ge3O12) BGO single crystal scintillators under x-ray excitation. Results will be of value in designing dual modality tomographic systems (PET/CT, SPECT/CT) based on a common scintillator crystal. All scintillating crystals have dimensions of 10 × 10 × 10 cm3 are non-hygroscopic exhibiting high radiation absorption efficiency in the energy range used in medical imaging applications. The comparative investigation was performed by determining the x-ray luminescence efficiency (emitted light flux over incident x-ray energy flux) in the range of x-ray energies employed in: (i) general x-ray imaging (40–140 kV, using a W/Al x-ray spectrum) and (ii) x-ray mammography imaging (22–49 kV, using a Mo/Mo x-ray spectrum). Additionally, light emission spectra of crystals at various x-ray energies were measured, in order to determine the intrinsic conversion efficiency and the spectral compatibility to optical photon detectors incorporated in medical imaging systems. The light emission performance of LYSO:Ce scintillator studied was found very high for x-ray imaging.

X-ray fluorescence spectroscopy from ions at charged vapor/water interfaces

X-ray fluorescence spectra from monovalent ions (Cs+) that accumulate from dilute solutions to form an ion-rich layer near a charged Langmuir monolayer are presented. For the salt solution without the monolayer, the fluorescence signals below the critical angle are significantly lower than the detection sensitivity and only above the critical angle signals from the bulk are observed. In the presence of a monolayer that provides surface charges, strong fluorescence signals below the critical angle are observed. Ion density accumulated at the interface are determined from the fluorescence. The fluorescent spectra collected as a function of incident x-ray energy near the LIII edge yield the extended absorption spectra from the ions, and are compared to recent independent results. The fluorescence data from divalent Ba2+ with and without monolayer are also presented.

Parameters governing gold nanoparticle X-ray radiosensitization of DNA in solution

Radiosensitization by gold nanoparticles (GNP) is a promising approach for improving radiotherapy. We report herein the results of an investigation of three key-parameters governing such radiosensitization in DNA, namely, DNA:GNP molar ratio, GNP diameter and incident X-ray energy. We performed irradiations with a clinical orthovoltage source and tested concentration ratios up to 1:1, five sizes of GNP from 8 to 92 nm and six effective X-ray energies from 14.8 to 70 keV. The most efficient parameters are found to be large-sized GNP, high molar concentration and 50-keV photons, which could potentially result in a dose enhancement factor of 6. The relevance of such parameters as regards the development of future therapeutic applications is discussed. To the best of our knowledge, this study constitutes the first report of systematic data on radiosensitization by GNP.

Investigation of Luminescence Properties of Lu$_{2}$SiO$_{5}$:Ce (LSO) Powder Scintillator in the X-Ray Radiography Energy Range

This paper investigates the light emission efficiency of Lu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> :Ce (LSO) powder scintillator under conditions employed in projection X-ray imaging. Although single-crystal LSO has been thoroughly studied in medical imaging energies, the efficiency of powder LSO has not been previously investigated experimentally under X-ray medical radiography conditions. For this purpose, three scintillating screens with a coating thickness of 63.4, 108.4, and 172.5 mg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> were prepared by sedimentation of LSO powder. Measurements of luminescence efficiency (emitted light energy flux over incident X-ray energy flux) were performed in the radiographic X-ray tube voltages range (40-140 kVp). Parameters related to X-ray detection, i.e., quantum detection efficiency, energy absorption efficiency and probability of K-fluorescence reabsorption were calculated. A theoretical model, describing radiation and light transfer, was employed to fit luminescence experimental data and to estimate values of the intrinsic conversion efficiency and of the light attenuation coefficients of the scintillating screens. The spectral compatibility of the LSO powder scintillator to various optical detectors used in digital radiography was also determined by performing light emission spectrum measurements and by taking into account the spectral sensitivity of electronic optical detectors. Maximum values of LSO luminescence efficiency were observed within the range from 40 to 60 kVp. The 172.5 mg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> thick screen was found more efficient than the two thinner screens examined in our experiments. The intrinsic conversion efficiency of LSO (0.10) was found close to that of currently employed phosphors. Spectral compatibility data of LSO screens indicates that LSO may be efficiently coupled to optical detectors used in digital radiography imaging systems.

Element-specific hard x-ray diffraction microscopy

An element-specific coherent x-ray imaging technique using anomalous x-ray scattering in the hard x-ray region was first demonstrated. Coherent x-ray diffraction patterns of a sample composed of 500-nm-thick Ni and Cu layers were measured at incident x-ray energies around the $\text{Ni}\text{ }K$ absorption edge. Non-center-symmetric diffraction patterns due to anomalous scattering phenomenon in the hard x-ray region were observed. Symmetricity of the diffraction pattern was quantitatively analyzed by numerically simulating the x-ray wave field behind the sample position using the Rytov approximation. By calculating the difference between the intensities of reconstructed images of different energies, an image of the Ni layers could be derived although it was not enough to identify precisely. This method is widely applicable to nondestructive analysis of nanometer-scale elemental distribution of materials buried within thick and high-$Z$ samples.

Shift-and-add tomosynthesis of a finger joint by X-ray dark-field imaging: Difference due to tomographic angle

A tomogram of a finger joint showing articular cartilage was generated based on X-ray dark-field imaging (XDFI) using the shift-and-add tomosynthesis algorithm. The experiment was performed at beamline 14B of the Photon Factory in Tsukuba, Japan, using synchrotron X-rays from a vertical wiggler. The incident X-ray energy was 36.0 keV. The X-ray optics for XDFI comprised two Si crystals: an asymmetric cut Si (2 2 0) monochromator–collimator and a 1.1-mm thick Si (2 2 0) Laue-case analyzer. The object was an intact cadaveric proximal interphalangeal joint fixed in formalin. Raw projection data were acquired by XDFI in a total of 41 views through an angle of 20° in 0.5° increments. The object and detector were synchronously rotated such that the fulcrum plane in the object and detector plane remained parallel. The X-ray dose for one piece of raw projection data was set to one-eleventh of that for one standard projection image by XDFI. Eleven views through an angle of 10° in increments of 1° of all 41 appropriately shifted raw projection data were added to produce arbitrary tomograms parallel to the fulcrum plane. We obtained a clear tomogram of the finger joint including the articular cartilage with the moderate artifact peculiar to tomosynthesis. Consequently, arbitrary tomograms can be obtained for the same X-ray dose as that received for one standard projection image by XDFI. The fact that an inner structure such as articular cartilage, which is invisible to conventional X-ray imaging methods, has been visualized on a tomogram with preserved refraction-enhanced contrast, is of considerable significance to clinical medicine.

Luminescence Emission Properties of $({\rm Lu},{\rm Y})_{2}{\rm SiO}_{5}$:Ce (LYSO:Ce) and $({\rm Lu},{\rm Y}){\rm AlO}_{3}$:Ce (LuYAP:Ce) Single Crystal Scintillators Under Medical Imaging Conditions

LYSO:Ce and LuYAP:Ce are single crystal non-hygroscopic scintillators of high density, high light yield and short decay time, which have been successfully used in small animal PET imagers. In the present study, the luminescence emission properties of (Lu0.9, Y0.1)2SiO5:Ce (LYSO:Ce) and (Lu0.7, Y0.3)AIO3:Ce (LuYAP:Ce) crystals were investigated for use in X-ray medical imaging. Both crystals had dimensions of 2 times 2 times 8 mm3, with all surfaces polished. Evaluation was performed by determining the X-ray luminescence efficiency (XLE) (emitted light energy flux over incident X-ray energy flux) and the detector optical gain (DOG) (emitted light photons per incident x-ray photon) in a wide range of X-ray energies employed in mammography (22-49 kVp) and in general X-ray imaging (50-140 kVp). Measurements were performed using an experimental set-up based on a photomultiplier coupled to an integration sphere. The emission spectrum under X-ray excitation was measured using an optical grating monochromator to determine the spectral compatibility to various optical photon detectors incorporated in medical imaging detectors. Optical characteristics such as transmission and absorption spectra were investigated in addition to the scintillation properties. The light emission performance of the two scintillation materials studied was found adequately high for X-ray imaging.

Emission statistics of X-ray induced photoelectrons and its comparison with electron- and ion-induced electron emissions

The emission statistics of secondary electrons from a gold metal surface induced by monochromatic X-rays is studied by Monte Carlo simulations. The number distributions of emitted electrons n and their mean values γ are calculated systematically for incident photon energies from 1 to 100keV. The results are compared with recent experimental results measured at the SPring-8 X-ray beam facility (BL15XU). We found that both theoretical and experimental results of the statistical number distributions of secondary electrons can be reproduced fairly well by Polya-type functions, showing small probabilities for one-electron emission (n=1) and broad distributions for high-n emission. In contrast, these features can never be reproduced by Poisson statistics. Furthermore, calculated emission yields γ are found to depend rather weakly on the incident X-ray energy. These results indicate clearly that fast photoelectrons produced by high-energy X-rays are responsible for high-n emission although the photoionization cross sections are considerably smaller at higher X-ray energies. Simulations are also extended to electron and ion bombardments, and a comprehensive comparison between X-rays and charged particle impacts is given for the emission statistics of electrons from a metal surface.

Momentum-Specified OxygenKResonant Inelastic X-ray Scattering for Cuprates

The present study theoretically shows that oxygen K resonant inelastic X-ray scattering (O K RIXS) in cuprates gives momentum-resolved information on the occupied and unoccupied electronic states. By tuning the incident X-ray energy carefully, we can select the k point where the X-ray absorption process as well as the subsequent X-ray emission process actually takes place. The requirement for the momentum specification is that the core-hole potential effect in the intermediate state of the RIXS should be negligible. This requirement is fulfilled only for the O K RIXS in the case of cuprates.

Structural Analysis of Polyelectrolyte Film Absorbing Metal Ion by SAXS Utilizing with X-ray Anomalous Dispersion Effect

A distribution of Cu ions in polyelectrolyte film (Nafion) is directly observed with a small-angle X-ray scattering (SAXS) method utilizing an X-ray anomalous dispersion effect. A partial structure factor of the Cu ions, GAA(q), can be derived from the SAXS profiles obtained by scanning the incident X-ray energy around the Cu K absorption edge. GAA(q) has two peaks, indicating that the Cu ions hierarchically distribute in Nafion film. In addition, a standard SAXS also shows that Nafion film has a hierarchical structure. These results mean that the Cu ions locate in the domain where the hydrophilic bases aggregate.

Luminescence Properties of $({\rm Lu},{\rm Y})_{2}{\rm SiO}_{5}:{\rm Ce}$ and ${\rm Gd}_{2}{\rm SiO}_{5}:{\rm Ce}$ Single Crystal Scintillators Under X-Ray Excitation for Use in Medical Imaging Systems

The luminescence of (Lu,Y)2SiO5 : Ce (LYSO:Ce) and Gd2SiO5:Ce (GSO:Ce) crystals was studied for use in tomographic medical x-ray imaging. LYSO:Ce and GSO:Ce are high density (7.1 g/cm3 and 6.71 g/cm3 respectively), high atomic number (71 for Lu and 64 for Gd), non-hydroscopic, and short decay time (40 ns and 60 ns respectively) scintillators. Evaluation was performed by determining: 1) the luminescence efficiency (LE) (emitted light energy flux over incident x-ray energy flux) in x-ray energies employed in general x-ray imaging (40-140 kVp) and in mammographic x-ray imaging (22-49 kVp), 2) the light emission spectrum, determined at various x-ray energies (22-140 kVp), and 3) the spectral compatibility to optical photon detectors, incorporated in medical imaging systems. Both scintillation materials exhibited adequately high LE in the x-ray diagnostic energy range, with LYSO:Ce's LE being distinctively higher. LYSO:Ce and GSO:Ce were found most compatible with the S-20 photocathode (0.9 for both materials) and adequately compatible to the amorphous silicon photodiode (0.74 for both materials), incorporated in many digital x-ray detectors

An iterative three-dimensional electron density imaging algorithm using uncollimated Compton scattered x rays from a polyenergetic primary pencil beam

X-ray film-screen mammography is currently the gold standard for detecting breast cancer. However, one disadvantage is that it projects a three-dimensional (3D) object onto a two-dimensional (2D) image, reducing contrast between small lesions and layers of normal tissue. Another limitation is its reduced sensitivity in women with mammographically dense breasts. Computed tomography (CT) produces a 3D image yet has had a limited role in mammography due to its relatively high dose, low resolution, and low contrast. As a first step towards implementing quantitative 3D mammography, which may improve the ability to detect and specify breast tumors, we have developed an analytical technique that can use Compton scatter to obtain 3D information of an object from a single projection. Imaging material with a pencil beam of polychromatic x rays produces a characteristic scattered photon spectrum at each point on the detector plane. A comparable distribution may be calculated using a known incident x-ray energy spectrum, beam shape, and an initial estimate of the object's 3D mass attenuation and electron density. Our iterative minimization algorithm changes the initially arbitrary electron density voxel matrix to reduce regular differences between the analytically predicted and experimentally measured spectra at each point on the detector plane. The simulated electron density converges to that of the object as the differences are minimized. The reconstruction algorithm has been validated using simulated data produced by the EGSnrc Monte Carlo code system. We applied the imaging algorithm to a cylindrically symmetric breast tissue phantom containing multiple inhomogeneities. A preliminary ROC analysis scores greater than 0.96, which indicate that under the described simplifying conditions, this approach shows promise in identifying and localizing inhomogeneities which simulate 0.5 mm calcifications with an image voxel resolution of 0.25 cm and at a dose comparable to mammography.

Wide-band imaging spectrometer with scintillator-deposited charge-coupled device

We report on a new photon-counting detector possessing unprecedented spatial resolution and moderate spectral resolution for 0.1– 100 keV X-rays. It consists of an X-ray charge-coupled device (CCD) and a scintillator. The scintillator is directly coupled to the back surface of the X-ray CCD. Low-energy X-rays below 10 keV can be directly detected by the CCD. The majority of hard X-rays above 10 keV pass through the CCD but can be absorbed by the scintillator, generating visible light photons. Since CCDs have a moderate detection efficiency, visible light photons can be detected by the CCD. We coupled the needlelike CsI(Tl) on the front surface of the back-illuminated (BI) CCD. We evaluated the spectroscopic performance for hard X-rays at the synchrotron facility, SPring-8, and found a good linear relationship between the incident X-ray energy and the pulse height up to 80 keV. The on-axis image of the hard X-ray telescope, supermirror, was measured by our device at 40 keV. A sharp core and the wing structure can be clearly imaged and high imaging capability of the SD-CCD can be demonstrated.

The charge cloud shape generated by a Cu-K X-ray photon inside the CCD

An X-ray photon generates a finite size of charge cloud in a charge-coupled device (CCD). It forms various pattern of events, depending on the X-ray interaction position on the CCD. The mesh experiment is the only available method to determine the charge cloud shape, which depends on the incident X-ray energy. We employed a metal mesh of 13 μ m in thickness with many holes of 2 μ m diameter periodically spaced of 48 μ m . The valid energy range depends on the X-ray opacity of the metal. It has been thought to be limited to below 7.0 keV for the gold mesh employed. The new method is introduced to roughly estimate the X-ray interaction position to reduce ‘background’ events so that the energy range can be extended to 8.2 keV. Applying our method to Cu-K X-rays (8.04 keV), we measure the charge cloud size generated by Cu-K X-rays and find it to be bigger than that expected from a diffusion model in the depletion region of the CCD. This is due to the fact that the CCD employed has a depletion region of about 40 μ m that is much shorter than the attenuation length of Cu-K X-rays in Si.

Correlative microXRF and optical immunofluorescence microscopy of adherent cells labeled with ultrasmall gold particles

Synchrotron-based X-ray fluorescence microscopy (microXRF) is a powerful tool to study the two-dimensional distribution of a wide range of biologically relevant elements in tissues and cells. By growing mouse fibroblast cells directly on formvar-carbon coated electron microscopy grids, microXRF elemental maps with well-defined subcellular resolution were obtained. In order to colocalize the elemental distribution with the location of specific cellular structures and organelles, we explored the application of a commercially available secondary antibody conjugated to FluoroNanogold, a dual-label that combines a regular organic fluorophore with a 1.4 nm Au-cluster as xenobiotic label for microXRF imaging. Adherent mouse fibroblast cells were grown on silicon nitride windows serving as biocompatible XRF support substrate, and labeled with FluoroNanogold in combination with primary antibodies specific for mitochondria or the Golgi apparatus, respectively. Raster scanning of the in-air dried cells with an incident X-ray energy of 11.95 keV, sufficient to ensure excitation of the Au Lα line, provided two-dimensional maps with submicron resolution for Au as well as for most biologically relevant elements. MicroXRF proved to be sufficiently sensitive to image the location and structural details of the Au-labeled organelles, which correlated well with the subcellular distribution visualized by means of optical fluorescence microscopy.

Excitations of lithium ammonia complexes studied by inelastic x-ray scattering

We have carried out high-resolution inelastic x-ray scattering measurements of the excitations of lithium dissolved in ammonia. The incident x-ray energy was 21.6 keV and the resolution was about 2 meV. Several different excitations are observed in the energy range of 0-60 meV (0-500 cm(-1)). In addition to acoustic phonons at low energies, we see excitations that are associated with vibrations of Li(NH3)4+ complexes. We examined these excitations as a function of momentum transfer, lithium concentration, temperature, and state of the system (solid versus liquid). Data are compared with Hartree-Fock and density-functional theory calculations of the excitations of this complex, which agree well with the measured excitation energies.

Nanostructured coatings of metal containing diamond-like carbon films deposited by femtosecond pulsed laser ablation

Nanocrystallized metal (Me)/carbon-based films (a-C:Me) have been deposited by femtosecond pulsed laser ablation (PLD). The films have been obtained by ablating alternatively and sequentially graphite and metallic targets in vacuum conditions, with an amplified femtosecond Ti:sapphire laser (wavelength 800 nm, pulsed duration of 150 fs). The metals selected, regarding of their opposite chemical affinity with carbon, are nickel and tantalum. The deposition procedure allowed to control accurately the metal concentration introduced in the carbonaceous matrix, in the 15 at.% range. The composition and nanostructure of the films have been characterized by X-ray photoelectron spectroscopy, grazing incident angle X-ray diffraction and energy filtered transmission electron microscopy. The nanocomposite coatings contain metallic clusters in the 10–250 nm size range, embedded in the carbonaceous network. The interface between the metallic clusters and the carbon matrix depends on the chemical affinity of the metal in respect to carbon.

Nuclear Resonant Scattering of Synchrotron Radiation by158Gd

An energy tunable synchrotron radiation (SR) source has a high capability to perform the nuclear resonant scattering experiments with various Mossbauer elements. So far, most of studies of nuclear resonant scattering have been conducted by using several isotopes having a Mossbauer level below 50 keV. When the SR source extends over the X-rays of higher energy region, the number of available Mossbauer isotopes increases remarkably. In this view point, the third generation SR facility such as SPring-8 (Hyogo, Japan) is very suitable as the X-ray source for high-energy nuclear resonant scattering experiments. Actually, the standard undulator beamline of SPring-8 can realize a high photon flux even for over 50 keV. It is due to the high electronic acceleration energy of 8.0 GeV. In this paper, we report the nuclear excitation of a gadolinium (Gd) isotope by SR in order to explore the prospective implementability of high energy nuclear resonant scattering using SR. As is shown in Table I, gadolinium is a unique Mossbauer element which has nine nuclear resonance energies in six isotopes, and all observable Mossbauer levels lie in the high energy region over 50 keV [in Table I, 2.18(3) signifies 2:18 0:03 etc.]. As for the four lighter isotopes of Gd, Gd, Gd, and Gd, Europium parent isotopes can be used as the radioactive Mossbauer source. In particular, the Mossbauer level of Gd can also be populated by electron capture (EC) decay of Tb. As for the two heavier isotopes of Gd and Gd, they have no appropriate radioactive parent nuclide. Therefore, the coulomb excitation technique for Gd and Gd or the in-beam Gd(n, ) neutron capture reactions for Gd has been used. In contrast, if the SR is used as a Mossbauer source for Gd, all available Mossbauer levels can be excited selectively by adjusting the incident X-ray energy. In present experiment, we have observed the excitation of the 79.5-keV nuclear level of Gd. The experiments were performed at the JAERI undulator beamline (BL11XU) of SPring-8. The experimental optics is shown in Fig. 1. The electron current of the storage ring was constantly kept up 100mA at 8GeV by the ‘‘top-up’’ operation. The ‘‘84 (4-bunch train)’’ operation mode of the storage ring was used for the measurement. As is shown in the upper left side of Fig. 1, this mode consists of 84 bunch-trains equally separated by a 51.1-ns interval, and each bunch-train is composed of consecutive 4 bunches. The undulator gap was tuned so that the thirteen harmonic of the undulator radiation corresponds to the resonance energy of Gd (79.51 keV). The X-rays from the undulator were monochromatized around the resonance energy of Gd by the liquid-N2-cooled double-crystal Si(333) reflections, 8,9) whose angle had been calibrated precisely by measuring the Sn nuclear resonance energy [23.879478(18) keV] with SnO2 powder. Adding to Si(333) reflection, Si(111) reflection, whose energy is 26.5 keV, also passes through the monochromator. Moreover, the flux intensity is so strong that the Si avalanche photodiode (APD) detector is saturated. Therefore, an Al (thickness is 25mm) absorber was inserted in order to cut the X-rays from Si(111) reflection. With this thickness of Al, the 26.5-keV X-rays were reduced to 0.0043%, while 25.9% of the 79.51-keV X-rays were transmitted. The incident beam was limited to 0:5 2mm with a slit, and was directed to a non-enriched Gd2O3 powder sample. A Si-APD was placed beneath the sample to observe the incoherent nuclear resonant scattering. The APD element (fabricated by Hamamatsu Photonics) had a detection area of 7.1mm (3mm in diameter) and a depletion layer of 30 mm. The X-ray detection efficiency was about 0.15% at 79.51 keV. To find the resonance excitation of Gd, delayed signals after 9 ns from the first prompt incident X-ray pulses were counted. By changing the Bragg angle of Si(333) monochromator, the incident X-ray energy was changed and a resonance peak was searched at room temperature (298K). The result of the energy scan is shown in Fig. 2. The nuclear resonant scattering from Gd was clearly observed, and the average count rate of the resonance peak was 0.4 count/s. Observed delayed counts were normalized with the incident X-ray flux on the sample. From the result of the Gaussian curve fitting, the measured nuclear resonance energy of Gd was evaluated to be Table I. Mossbauer isotope characteristics of gadolinium.

Structures of Co 90Fe 10/Cu multilayers determined by X-ray anomalous scattering measurements

The structures of Co 90Fe 10/Cu multilayers were investigated by X-ray anomalous scattering techniques. It is found that the Cu/Co 90Fe 10 and Co 90Fe 10/Cu interfaces have different thermal behavior upon annealing. For the as-deposited sample, a Cu 2Co intermixing region exists at the Cu/Co 90Fe 10 interface, while the Co 90Fe 10/Cu interface is sharp. There exists only a Co 70Fe 30 sublayer close to the interface in the Co 90Fe 10 layer. After annealing at 285 °C for 2 h, more Cu react with Co to create a Cu 3Co intermixing region at the Cu/Co 90Fe 10 interface, while no obvious change is observed at the Co 90Fe 10/Cu interface. The X-ray diffuse scattering measurements performed with the incident X-ray energy close to Fe, Co and Cu K edges, respectively, revealed an island growth mechanism of the multilayers.

The use of selected monochromatic X-rays to induce a cascade of gamma transitions from the 31-year nuclear isomer to the 4 second isomeric state of Hf-178

The Hf-178m2 nuclear spin isomer stores 2.45 MeV of energy for a half life of 31 years. Unperturbed, such nuclei radiate away the stored energy through the emission of gamma photons from electromagnetic (EM) transitions occurring within the nuclei. It has been shown that the irradiation of samples containing such nuclei with pulsed X-rays can accelerate the rate of the EM transitions by relaxing the selection rules upon changes of angular momenta. To date, most work has been done with incident X-ray energies between 9 and 10 keV, and in such cases the acceleration of the rate of gamma emission is immediate. Reported here is a channel for deexcitation excited by more energetic X-rays that results in a cascade of gamma transitions that includes a 4 second statistical time lag. This more protracted release of the energy stored in samples of the Hf-178m2 nuclear isomers encourages consideration of potential mechanical and thermal applications.