摘要:

Many experiments in nuclear and particle physics would benefit from a device capable of detecting nonionizing events with a low energy threshold. We report on experimental tests of a low‐temperature detector based on the registration of nonequilibrium phonons. The device is composed of a silicon single crystal with superconducting tunnel junctions (Al/Al2O3/Al) evaporated onto its surface. In first experiments a 20×10×3 mm3crystal at an operating temperature ofT= 0.37 K was tested with 5.5‐MeV &agr; particles. Pulse‐height analysis and the timing of pulses in different junctions is shown to yield position and energy resolution. An energy threshold of 250 keV was estimated for absorption anywhere in the crystal. A position resolution of 0.6 mm was determined over a sensitive length of 10 mm. Though phonon focusing effects were registered in the vicinity of the [001] direction, scattered and reflected phonons dominate the signals in the general case. The experimental results are discussed in terms of ballistic and quasidiffuse phonon propagation. To improve the energy threshold, the operating temperature was lowered to 60 mK in order to increase junction sensitivity and a weak thermal coupling of the absorber to the heat bath was introduced to force phonons to leave the crystal via the detecting junctions. Si crystals of up to 20×20×10 mm3size were used in these second experiments. With a 4 cm3crystal a threshold of 270 keV was obtained. The performance in these experiments at 60 mK, however, was limited by exceptionally high values of the normal conducting junction resistances. With a reasonable value of this parameter it should be possible to realize an energy threshold of 1 keV with absorbers of a volume of several cm3.

ISSN:0021-8979    

DOI:10.1063/1.347233

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The problems of an electric and a magnetic dipole placed at the center of an open spherical, perfectly conducting cavity are analytically investigated. Numerical applications illustrate graphically the theoretical results.

ISSN:0021-8979    

DOI:10.1063/1.348775

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The diffusion of an external magnetic field through a toroidal conducting shell is studied under the assumption of a small aspect ratio. The external magnetic field can have an arbitrary field index and magnetic flux on the minor axis of the torus. The diffused field, field index, magnetic flux, and wall current are computed analytically and compared with the numerical results from the TRIDIF code. The analytical and numerical results are in good agreement. Measurements in the toroidal chamber of the NRL modified betatron gave a delay time of 34 &mgr;sec, which is less than 10% from the theoretical prediction of 37 &mgr;sec.

ISSN:0021-8979    

DOI:10.1063/1.348776

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

This paper describes the injection and trapping of a high‐current space‐charge‐dominated electron beam into a closed cyclic accelerator. A method has been developed to transport externally generated beams into a strong‐focusing betatron using a pulsedE×Bdrift. The method requires modest electric fields, gives good trapping efficiency, provides continuous focusing of high‐perveance beams, has tolerance to imperfections, and has the potential ability to maintain the emittance of the injected beam. The accelerator was a racetrack machine with an 8‐m circumference. Forty solenoid coils with alternating field polarity provided strong focusing. The 220‐A, 500‐keV injected beam entered the machine through a 2.5‐cm‐diam shielded transport tube. A pulsed electric field captured a 30‐ns slice of the beam− the system trapped most of the current that had completed a full circuit. The main problem encountered in the experiments was a rapid loss of beam electrons during and after trapping. Electrons traveled an average of only two revolutions (16 m). Experimental observations and theoretical studies implied that magnetic dipole field errors in the injector were the primary cause of beam loss.

ISSN:0021-8979    

DOI:10.1063/1.348777

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The influence of the perturbed reference wave in electron holography is considered for the case of static electromagnetic microfields, whose extension around the observed specimen cannot be neglected. These microfields are called ‘‘long‐range’’ to distinguish them from the ‘‘short‐range’’ ones, whose extension is strictly limited within the object wave and hence do not perturb the reference wave. Optical reconstructions of experimental holograms of simple electrostatic or magnetic long‐range fields have been modeled and simulated. The results indicate that perturbation effects must be taken into account when long‐range microfields are investigated by electron holography; it is also shown that their influence can be minimized by increasing the interference distance between the object and reference wave.

ISSN:0021-8979    

DOI:10.1063/1.348970

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The intrinsic power efficiency of the atomic xenon laser depends upon the electron density because of the mixing of the laser levels by electron collisions while the electron density in high‐pressure particle‐beam excited plasmas increases with increasing gas temperature. Therefore, in order to reduce the amount of electron collisional mixing when operating at high‐energy loadings (≳100’s J/1‐atm) mixtures having a high‐heat capacity are required. In particle‐beam excited Ar/Xe mixtures, which typically yield the highest intrinsic laser efficiencies, increasing the gas pressure to increase the heat capacity is not always practical due to the high‐stopping power of the gas mixture. For this reason we have experimentally and theoretically investigated adding He to Ar/Xe mixtures in studies of a fission‐fragment excited atomic xenon laser. Adding He increases the heat capacity without appreciably perturbing the favorable kinetics resulting in efficient operation of the laser in Ar/Xe mixtures. We find that when adding He to Ar/Xe mixtures the dominant laser transition switches from 1.73 to 2.03 &mgr;m without significantly decreasing the efficiency. The laser pulse length also increases, an effect attributed to a lowering of both the electron temperature and gas temperatures.

ISSN:0021-8979    

DOI:10.1063/1.348752

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Chlorine incorporated into silica glasses by sintering porous soot rods under Cl2/He atmosphere or by fabricating with plasma methods were found to be mostly as ≡SiCl, which gives rise to an optical absorption tail above &bartil;7.5 eV. The cross section of the optical absorption at &bartil;7.77 eV (160 nm) was found to be 1.14×1020cm2. This value was almost the same as that of ≡SiCl in SiCl4molecule in the gas phase. A minor fraction (10−3∼10−1) was found to be present as Cl2molecule, which gives the absorption band at 3.8 eV.

ISSN:0021-8979    

DOI:10.1063/1.348753

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The use of diffractive output coupling and potentially easy fabrication make unstable resonators suitable for extreme ultraviolet and soft x‐ray lasers. We illustrate the design of an unstable resonator cavity for a proposed electron collisionally excited Ni‐like Mo laser at 191 A˚. Saturated output in a near‐diffraction limited beam is predicted in a six‐pass synchronously pumped cavity with a gain‐length product of 6.

ISSN:0021-8979    

DOI:10.1063/1.348754

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Characteristics of Fe‐doped semi‐insulating (SI) InP layers with overgrown Zn‐dopedp‐type layers have been investigated by scanning electron microscope, secondary‐ion mass spectrometry (SIMS), and capacitance‐voltage (C‐V) and current‐voltage (I‐V) measurements. Resistivity of the structures determined from the measuredI‐Vcharacteristics was found to be strongly dependent on the Zn doping concentration. The SIMS depth profiles showed Zn accumulation at the SI/p‐InP interface and the peak concentration of the Zn accumulation increased with the doping level and overgrowth time of thep‐InP layers. This accumulation of Zn at the SI/p‐InP interface correlated with reduction in SI layer resistivity. Accumulation of Zn at the SI/p‐InP interface may be minimized by short growth time with low or medium doping ofp‐InP layers. These growth conditions resulted in high SI layer resistivity. Possible mechanisms for the accumulation of Zn are discussed.

ISSN:0021-8979    

DOI:10.1063/1.348755

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The effects of pump polarization on the performance of synchronously pumped mode‐locked dye lasers have been investigated. A semiclassical model is derived and the numerical results are compared with experimental data. The theoretical pump polarization dependencies of the pulse shape, pulse width, peak power, and pulse delay have been determined, and experimental data obtained with an argon‐pumped rhodamine 6G dye laser are in good agreement with the theory.

ISSN:0021-8979    

DOI:10.1063/1.348756

出版商:AIP    

年代:1991

数据来源: AIP