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Features

• Spectroscopy from 3 keV up
• Wide range of sizes
• High resolution - good peak shape
• No slow pulses
• Thin front contact
• High energy rate capability

Description

The Low Energy Germanium Detector (LEGe) represents a new concept in Germanium Detector Geometry with major advantages over conventional planar or coaxial detectors in many applications. The LEGe detector is fabricated with a thin front contact. The rear contact is of less than full area and thus the capacitance of the detector is less than that of a planar device of similar size. Since preamplifier noise is a function of detector capacitance, the LEGe affords lower noise and consequently better resolution at low and moderate energies than any other detector geometry. Unlike grooved planar detectors, there is little dead germanium beyond the active region. This, and the fact that the side surface is charge collecting rather than insulating, results in fewer long-rise time pulses with improved count rate performance and peak-to-background ratios.

The LEGe detector is available with active areas from 50 mm² to 38 cm² and with thicknesses ranging from 5 to 25 mm. For applications involving moderate gamma-ray energies, the LEGe may well outperform an expensive large volume coaxial detector. The efficiency curve given below illustrates the performance of a typical LEGe detector.

To take full advantage of the low energy response of this intrinsically thin window detector, the LEGe is usually equipped with a thin Be window. For applications involving energies exclusively above 30 keV or so, the LEGe can be provided with a conventional 0.5 mm Aluminum window. Other window options are available. In any case, a wide range of CANBERRA cryostats is available to choose from.

Absolute Efficiency Curve for LEGe Detector with 2.5 cm Spacing Between Source and End Cap

PREAMPLIFIERS

CANBERRA offers the choice of RC feedback preamplifiers or pulsed -optical preamplifiers on its LEGe detectors. The pulsed-optical type preamplifier affords better resolution at low energies and at low total energy rates. The RC preamplifier gives better performance at high total energy rates. It is, therefore, used exclusively on larger detectors which have greater stopping power for high energy photons. Block diagrams and descriptions of the operating principles of each type are illustrated.

RC Feedback

Charge impulses from the detector result in an equivalent charge stored on the feedback capacitor (C_f), producing the step function . V_°=Q_in/C_f. R_f discharges C_f with the time constant 1/R_fC_f. This signal is differentiated by C₂-R₂ and amplified X2 or X10 by amp A₂.

Block Diagram - RC Preamp

OPTIONAL: INTEGRATED-TRANSISTOR RESET (I-TRP) PREAMPLIFIER

The Integrated-Transistor Reset Preamplifier is similar to the Pulsed-Optical Reset (POR) preamplifier. Instead of using an LED to reset the preamplifier, however, an integrated transistor switch does the job. The I-TRP is notably free from the spurious effects seen in POR preamps such as long recovery time associated with illumination of the FET. The short recovery lends itself to high rate applications and the FET itself contributes less noise resulting in better resolution, especially with short amplifier or pulse processor time constants.

PULSED-OPTICAL FEEDBACK PREAMPLIFIER

Charge impulses (Q_in) from the detector result in equivalent charge stored on feedback capacitor C_f producing step function V_o = Q_in/C_f. Subsequent impulses increase the output to a limit at which point the Schmitt trigger fires an LED directed at the FET. The light pulse momentarily shorts out the FET gate-source junction thus discharging C_f and resetting the preamplifier. An adjustable width monostable multivibrator provides a pulse which can be used to gate off the MCA during the transient reset time.

LEGe DETECTOR

General specifications and information Standard configuration includes:

• Vertical dipstick cryostat with Be window and 30 liter Dewar
• Pulsed optical feedback (P) or resistive feedback (R) preamplifier with cooled FET and 3 meter bias, signal and power cables

Resolution (FWTM) less than or equal to 2 times resolution (FWHM)

Model Number	Area (mm2)	Thickness (mm)	Be Window Thickness mm (mils)	Resolution (eV FWHM)		Preamp
				5.9 keV	122 keV
GL0055PS	50	5	0.05 (2)	145	500	PO
GL0055P	50	5	0.05 (2)	155	500	PO
GL0055R	50	5	0.05 (2)	175	500	RC
GL0110P	100	10	0.075 (3)	160	500	PO
GL0110R	100	10	0.075 (3)	180	500	RC
GL0210P	200	10	0.15 (6)	170	520	PO
GL0210R	200	10	0.15 (6)	195	520	RC
GL0213R	200	13	0.15 (6)	195	510	RC
GL0510P	500	10	0.15 (6)	200	550	PO
GL0510R	500	10	0.15 (6)	250	550	RC
GL0515R	500	15	0.15 (6)	250	550	RC
GL1010R	1000	10	0.5 (20)	300	650	RC
GL1015R	1000	15	0.5 (20)	300	650	RC
GL2015R	2000	15	0.5 (20)	400	700	RC
GL2020R	2000	20	0.5 (20)	400	700	RC
GL2820R	2800	20	0.5 (20)	450	750	RC
GL3825R	3800	25	0.5 (20)	475	750	RC

OPTIONS

Cryostat - see options and accessories price list.
Window -0.5 mm (20 mil) aluminum, no charge.
Window - 0.6 mm (24 mil) carbon composite, consult factory.

Block Diagram - Pulsed Optical Feedback Preamp