We present the design and performance of the multi-object fiber spectrographs for the Sloan Digital Sky Survey (SDSS) and their upgrade for the Baryon Oscillation Spectroscopic Survey (BOSS). Originally commissioned in Fall 1999 on the 2.5 m aperture Sloan Telescope at Apache Point Observatory, the spectrographs produced more than 1.5 million spectra for the SDSS and SDSS-II surveys, enabling a wide variety of Galactic and extra-galactic science including the first observation of baryon acoustic oscillations in 2005. The spectrographs were upgraded in 2009 and are currently in use for BOSS, the flagship survey of the third-generation SDSS-III project. BOSS will measure redshifts of 1.35 million massive galaxies to redshift 0.7 and Lyα absorption of 160,000 high redshift quasars over 10,000 deg2 of sky, making percent level measurements of the absolute cosmic distance scale of the universe and placing tight constraints on the equation of state of dark energy. The twin multi-object fiber spectrographs utilize a simple optical layout with reflective collimators, gratings, all-refractive cameras, and state-of-the-art CCD detectors to produce hundreds of spectra simultaneously in two channels over a bandpass covering the near-ultraviolet to the near-infrared, with a resolving power R = λ/FWHM ~ 2000. Building on proven heritage, the spectrographs were upgraded for BOSS with volume-phase holographic gratings and modern CCD detectors, improving the peak throughput by nearly a factor of two, extending the bandpass to cover 360 nm < λ < 1000 nm, and increasing the number of fibers from 640 to 1000 per exposure. In this paper we describe the original SDSS spectrograph design and the upgrades implemented for BOSS, and document the predicted and measured performances.

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The Multi-object, Fiber-fed Spectrographs for the Sloan Digital Sky Survey and the Baryon Oscillation Spectroscopic Survey

The Dark Energy Camera is a new imager with a 2.2-degree diameter field of view mounted at the prime focus of the Victor M. Blanco 4-meter telescope on Cerro Tololo near La Serena, Chile. The camera was designed and constructed by the Dark Energy Survey Collaboration, and meets or exceeds the stringent requirements designed for the wide-field and supernova surveys for which the collaboration uses it. The camera consists of a five element optical corrector, seven filters, a shutter with a 60 cm aperture, and a CCD focal plane of 250 micron thick fully-depleted CCDs cooled inside a vacuum Dewar. The 570 Mpixel focal plane comprises 62 2kx4k CCDs for imaging and 12 2kx2k CCDs for guiding and focus. The CCDs have 15 microns x15 microns pixels with a plate scale of 0.263 arc sec per pixel. A hexapod system provides state-of-the-art focus and alignment capability. The camera is read out in 20 seconds with 6-9 electrons readout noise. This paper provides a technical description of the camera's engineering, construction, installation, and current status.

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The dark energy camera

Charge-coupled devices (CCDs) have been fabricated on high-resistivity, n-type silicon. The resistivity, on the order of 10 000 /spl Omega//spl middot/cm, allows for depletion depths of several hundred micrometers. Fully depleted, back-illuminated operation is achieved by the application of a bias voltage to an ohmic contact on the wafer back side consisting of a thin in situ doped polycrystalline silicon layer capped by indium tin oxide and silicon dioxide. This thin contact allows for a good short-wavelength response, while the relatively large depleted thickness results in a good near-infrared response.

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Fully depleted, back-illuminated charge-coupled devices fabricated on high-resistivity silicon

We evaluated a commercial positron emission mammography (PEM) camera, the PEM Flex Solo II. This system comprises two 6 × 16.4 cm detectors that scan together covering up to a 24 × 16.4 cm field of view (FOV). There are no specific standards for testing this detector configuration. We performed several tests important to breast imaging, and we propose tests that should be included in standardized testing of PEM systems. Methods: We measured spatial resolution, uniformity, counting- rate linearity, recovery coefficients, and quantification accuracy using the system9s software. Image linearity and coefficient of variation at the edge of the FOV were also characterized. Anecdotal examples of clinical patient data are presented. Results: The spatial resolution was 2.4 mm in full width at half maximum for image planes parallel to the detector faces. The background variability was approximately 5%, and quantification accuracy and recovery coefficients varied within the FOV. Positioning linearity began at approximately 13 mm from the edge of the detector housing. The coefficient of variation was significantly higher close to the edge of the FOV because of limited sensitivity in these image planes. Conclusion: A reconstructed spatial resolution of 2.4 mm represented a significant improvement over conventional whole-body PET scanners and should reduce the lower threshold on lesion size and tracer uptake for detection in the breast. Limited-angle tomography and a lack of data corrections result in spatially variable quantitative results. PEM acquisition geometry limits sampling statistics at the chest-wall edge of the camera, resulting in high variance in that portion of the image. Example patient images demonstrate that lesions can be detected at the chest-wall edge despite variance artifacts, and fine structure is visualized routinely throughout the FOV in the focal plane. The PEM Flex camera should enable the functional imaging of breast cancer earlier in the disease process than whole-body PET.

https://jnm.snmjournals.org/content/jnumed/50/10/1666.full.pdf

Clinical Imaging Characteristics of the Positron Emission Mammography Camera: PEM Flex Solo II

The long patient port of a PET scanner tends to put stress on patients, especially patients with claustrophobia. It also prevents doctors and technicians from taking care of patients during scanning. In this paper, we proposed an 'open PET' geometry, which consists of two axially separated detector rings. A long and continuous field-of-view (FOV) including a 360° opened gap between two detector rings can be imaged enabling a fully 3D image reconstruction of all the possible lines-of-response. The open PET will become practical if iterative image reconstruction methods are applied even though image reconstruction of the open PET is analytically an incomplete problem. First we implemented a 'masked' 3D ordered subset expectation maximization (OS-EM) in which the system matrix was obtained from a long 'gapless' scanner by applying a mask to detectors corresponding to the open space. Next, in order to evaluate imaging performance of the proposed open PET geometry, we simulated a dual HR+ scanner (ring diameter of D = 827 mm, axial length of W = 154 mm × 2) separated by a variable gap. The gap W was the maximum limit to have axially continuous FOV of 3W though the maximum diameter of FOV at the central slice was limited to D/2. Artifacts, observed on both sides of the open space when the gap exceeded W, were effectively reduced by inserting detectors partially into unnecessary open spaces. We also tested the open PET geometry using experimental data obtained by the jPET-D4. The jPET-D4 is a prototype brain scanner, which has 5 rings of 24 detector blocks. We simulated the open jPET-D4 with a gap of 66 mm by eliminating 1 block-ring from experimental data. Although some artifacts were seen at both ends of the opened gap, very similar images were obtained with and without the gap. The proposed open PET geometry is expected to lead to realization of in-beam PET, which is a method for an in situ monitoring of charged particle therapy, by letting the beams pass through the gap. The proposed open PET geometry will also allow simultaneous PET/CT measurements of the same PET FOV as the CT FOV, in contrast to the conventional PET/CT where each FOV is separated by several tens of centimeters.

http://iopscience.iop.org/article/10.1088/0031-9155/53/3/015/pdf

A proposal of an open PET geometry.

We present the tomographic images and performance measurements of the LBNL positron emission mammography (PEM) camera, a specially designed positron emission tomography (PET) camera that utilizes PET detector modules with depth of interaction measurement capability to achieve both high sensitivity and high resolution for breast cancer detection. The camera currently consists of 24 detector modules positioned as four detector banks to cover a rectangular patient port that is 8.2/spl times/6 cm/sup 2/ with a 5 cm axial extent. Each LBNL PEM detector module consists of 64 3/spl times/3/spl times/30 mm/sup 3/ LSO crystals coupled to a single photomultiplier tube (PMT) and an 8/spl times/8 silicon photodiode array (PD). The PMT provides accurate timing, the PD identifies the crystal of interaction, the sum of the PD and PMT signals (PD+PMT) provides the total energy, and the PD/(PD+PMT) ratio determines the depth of interaction. The performance of the camera has been evaluated by imaging various phantoms. The full-width-at-half-maximum (FWHM) spatial resolution changes slightly from 1.9 mm to 2.1 mm when measured at the center and corner of the field of the view, respectively, using a 6 ns coincidence timing window and a 300-750 keV energy window. With the same setup, the peak sensitivity of the camera is 1.83 kcps//spl mu/Ci.

Characterization of the LBNL PEM camera

P-channel backside illuminated silicon charge-coupled devices (CCDs) were developed and fabricated on high-resistivity n-type silicon. The devices have been exposed up to 1 /spl times/ 10/sup 11/ protons/cm/sup 2/ at 12 MeV. The charge transfer efficiency and dark current were measured as a function of radiation dose. These CCDs were found to be significantly more radiation tolerant than conventional n-channel devices. This could prove to be a major benefit for space missions of long duration.

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Proton radiation damage in p-channel CCDs fabricated on high- resistivity silicon

We present an in situ calibration technique for the LBNL positron emission mammography (PEM) detector module that is capable of measuring depth of interaction (DOI). The detector module consists of 64 LSO crystals coupled on one end to a single photomultiplier tube (PMT) and on the opposite end to a 64 pixel array of silicon photodiodes (PD). The PMT provides an accurate timing pulse, the PDs identify the crystal of interaction, the sum provides a total energy signal and the /spl Gamma/=PD/(PD+PMT) ratio determines the depth of interaction. We calibrate using the /sup 176/Lu natural background radiation of the LSO crystals. We determine the relative gain (K) of the PMT and PD by minimizing the asymmetry of the /spl Gamma/ distribution. We determine the depth dependence from the width of the /spl Gamma/ distribution with optimal K. The performance of calibrated detector modules is evaluated by averaging results from 12 modules. The energy resolution is a function of depth ranging from 24% FWHM at the PD end to 51% FWHM at the PMT end, and the DOI resolution ranges from 6 mm FWHM at the PD end to 11 mm FWHM at the PMT end.

/pdf/calibration-of-a-pem-detector-with-depth-of-interaction-1ff52l7n8y.pdf

Calibration of a PEM detector with depth of interaction measurement

A new type of p-channel CCD constructed on high-resistivity n-type silicon was exposed to 12 MeV protons at doses up to 1x1011 protons/cm2. The charge transfer efficiency was measured as a function of radiation dose and temperature. We previously reported that these CCDs are significantly more tolerant to radiation damage than conventional n-channel devices. In the work reported here, we used pocket pumping techniques and charge transfer efficiency measurements to determine the identity and concentrations of radiation induced traps present in the damaged devices.

/pdf/proton-radiation-damage-in-high-resistivity-n-type-silicon-2qfxoc3z56.pdf

Proton radiation damage in high-resistivity n-type silicon CCDs

We present an in situ calibration technique for the LBNL positron emission mammography (PEM) detector module that is capable of measuring depth of interaction (DOI) The detector module consists of 64 LSO crystals coupled on one end to a single photomultiplier tube (PMT) and on the opposite end to a 64 pixel array of silicon photodiodes (PD) The PMT provides an accurate timing pulse, the PDs identify the crystal of interaction, the sum provides a total energy signal and the /spl Gamma/=PD/(PD+PMT) ratio determines the depth of interaction We calibrate using the /sup 176/Lu natural background radiation of the LSO crystals We determine the relative gain (K) of the PMT and PD by minimizing the asymmetry of the /spl Gamma/ distribution We determine the depth dependence from the width of the /spl Gamma/ distribution with optimal K The performance of calibrated detector modules is evaluated by averaging results from 12 modules The energy resolution is a function of depth ranging from 24 percent FWHM at the PD end to 51 percent FWHM at the PMT end, and the DOI resolution ranges from 6 mm FWHM at the PD end to 11 mm FWHM at the PMT end

/pdf/calibration-of-a-pem-detector-with-depth-of-interaction-107g8frwvf.pdf

G. Wang

Papers

Characterization of the LBNL PEM camera

Proton radiation damage in p-channel CCDs fabricated on high- resistivity silicon

Calibration of a PEM detector with depth of interaction measurement

Proton radiation damage in high-resistivity n-type silicon CCDs

Calibration of a PEM detector with depth of interaction measurement - eScholarship