Light Response Function Research Articles

Detector Light Response Modeling for a Thick Continuous Slab Detector

We investigate a method to improve the position decoding for thick crystal versions (i.e., ≥8 mm) of the continuous miniature crystal element (cMiCE) PET detector by more accurately modeling the detector light response function (LRF). The LRF for continuous detectors varies with the depth of interaction (DOI) of the detected photon. This variation in LRF can result in a positioning error for two-dimensional positioning algorithms. We explore a method to improve positioning performance by deriving two lookup tables, corresponding to the front and back regions of the crystal. The DETECT2000 simulation package was used to investigate the light response characteristics for a 48.8mm by 48.8mm by 10 (8) mm slab of LSO coupled to a 64-channel, flat-panel PMT. The data are then combined to produce the two-dimensional light collection histograms. Light collection histograms that have markedly non-Gaussian distributions are characterized as a combination of two Gaussian functions, where each Gaussian function corresponds to a DOI region of the crystal. The results indicate that modest gains in positioning accuracy are achieved near the central region of the crystal. However, significant improvements in spatial resolution and positioning bias are achieved for the corner section of the detector.

Leaf area index is the principal scaling parameter for both gross photosynthesis and ecosystem respiration of Northern deciduous and coniferous forests

Data on net CO 2 exchange from eight forests in Denmark, Sweden, Finland and Iceland were used to analyse which factors were controlling photosynthesis and respiration. The forests consisted of different species ranging in climatic condition from temperate to subarctic. Only well mixed conditions were analysed ( u * > 0.3 m s -1 ). The parameters of a light response function showed strong seasonal variations with similar behaviour for all stands except for a beech forest where the development of a vigorous ground vegetation in spring affected the photosynthesis parameters differently as compared to the other forests. The beech forest also showed the highest respiration rates in the earlier part of the growing season in contrast to the other forests that showed maximum values in late part of July. The mean half-monthly nighttime respiration rates were well explained by an equation with one fitting parameter, the respiration rate at 10 °C, with an r 2 = 0.864 for all stands together. The difference between the stands concerning both photosynthesis and respiration parameters were largely explained by the differences in LAI. After normalizing for LAI, the only remaining correlation was between respiration and stand age. These results are promising for application of remote sensing for estimation of respiration as well as gross primary productivity from forests. DOI: 10.1111/j.1600-0889.2007.00330.x

Detector Light Response Modeling for a Thick Continuous Slab Detector

We investigate a method to improve the position decoding for thick crystal versions (i.e., ≥8 mm) of the continuous miniature crystal element (cMiCE) PET detector by more accurately modeling the detector light response function (LRF). The LRF for continuous detectors varies with the depth of interaction (DOI) of the detected photon. This variation in LRF can result in a positioning error for two-dimensional positioning algorithms. We explore a method to improve positioning performance by deriving two lookup tables, corresponding to the front and back regions of the crystal. The DETECT2000 simulation package was used to investigate the light response characteristics for a 48.8 mm by 48.8 mm by 10 (8) mm slab of LSO coupled to a 64-channel, flat-panel PMT. The data are then combined to produce the two-dimensional light collection histograms. Light collection histograms that have markedly non-Gaussian distributions are characterized as a combination of two Gaussian functions, where each Gaussian function corresponds to a DOI region of the crystal. The results indicate that modest gains in positioning accuracy are achieved near the central region of the crystal. However, significant improvements in spatial resolution and positioning bias are achieved for the corner section of the detector.

Leaf area index is the principal scaling parameter for both gross photosynthesis and ecosystem respiration of Northern deciduous and coniferous forests

Data on net CO2 exchange from eight forests in Denmark, Sweden, Finland and Iceland were used to analyse which factors were controlling photosynthesis and respiration. The forests consisted of different species ranging in climatic condition from temperate to subarctic. Only well mixed conditions were analysed (u∗ > 0.3 ms-1). The parameters of a light response function showed strong seasonal variations with similar behaviour for all stands except for a beech forest where the development of a vigorous ground vegetation in spring affected the photosynthesis parameters differently as compared to the other forests. The beech forest also showed the highest respiration rates in the earlier part of the growing season in contrast to the other forests that showed maximum values in late part of July. The mean halfmonthly nighttime respiration rates were well explained by an equation with one fitting parameter, the respiration rate at 10 ◦C, with an r2 = 0.864 for all stands together. The difference between the stands concerning both photosynthesis and respiration parameters were largely explained by the differences in LAI. After normalizing for LAI, the only remaining correlation was between respiration and stand age. These results are promising for application of remote sensing for estimation of respiration as well as gross primary productivity from forests.

Effect of Number of Readout Channels on the Intrinsic Spatial Resolution Performance of a Continuous Miniature Crystal Element (cMiCE) Detector

In this work we investigate how the intrinsic spatial resolution varies with the number of digitized readout channels for a 64 (8×8) channel detector using a statistics-based positioning algorithm. We report results for both 6 mm and 8 mm thick crystals. Three channel reduction schemes are explored. The simplest scheme is row and column summing (R-C sum) of the photomultiplier tube channels. The second method is to only use channels with signals above a 1% threshold of the total signal (1% thres). The third method is to acquire a subset of PMT channels determined by the maximum signal channel (zone mask). The full width at half maximum (FWHM) intrinsic spatial resolution results for the central and corner sections of the detector are presented for each of the methods. All methods except R-C sum performed well for the central section of the detector. The 1% thres and zone mask schemes showed significant improvement for the corner section of the 6 mm thick crystal. All methods using a single depth look-up table had difficulty in the corner region for the 8 mm thick crystal. We believe this is caused by depth dependent edge effects on the light response function. Initial results using a depth dependent look-up table show improved positioning performance.

Proton Beam Energy Monitoring with Radiation-Hard Crystal Scintillators

Monitoring of the energy profiles of protons in different beam configurations is important not only for understanding the characteristics of proton beam but also for understanding the light output response of a scintillation crystal. The energy profile of a proton beam was measured with several types of crystal scintillators by using 35and 45-MeV proton beams from the MC-50 cyclotron at the Korea Institute of Radiological and Medical Sciences. Scintillation crystals of BGO, LYSO and GSO were used for the energy measurement of the proton beam. The pulse height spectrum and the energy resolution were measured for each crystal and compared with one another. The best energy resolution was 3 % FWHM with the GSO crystal. Different thicknesses of energy degraders were used with the 35and the 45-MeV proton incident energies to measure the light output response function of the BGO crystal.

Depth of interaction decoding of a continuous crystal detector module

We present a clustering method to extract the depth of interaction (DOI) information from an 8 mm thick crystal version of our continuous miniature crystal element (cMiCE) small animal PET detector. This clustering method, based on the maximum-likelihood (ML) method, can effectively build look-up tables (LUT) for different DOI regions. Combined with our statistics-based positioning (SBP) method, which uses a LUT searching algorithm based on the ML method and two-dimensional mean–variance LUTs of light responses from each photomultiplier channel with respect to different gamma ray interaction positions, the position of interaction and DOI can be estimated simultaneously. Data simulated using DETECT2000 were used to help validate our approach. An experiment using our cMiCE detector was designed to evaluate the performance. Two and four DOI region clustering were applied to the simulated data. Two DOI regions were used for the experimental data. The misclassification rate for simulated data is about 3.5% for two DOI regions and 10.2% for four DOI regions. For the experimental data, the rate is estimated to be ∼25%. By using multi-DOI LUTs, we also observed improvement of the detector spatial resolution, especially for the corner region of the crystal. These results show that our ML clustering method is a consistent and reliable way to characterize DOI in a continuous crystal detector without requiring any modifications to the crystal or detector front end electronics. The ability to characterize the depth-dependent light response function from measured data is a major step forward in developing practical detectors with DOI positioning capability.

Partitioning European grassland net ecosystem CO 2 exchange into gross primary productivity and ecosystem respiration using light response function analysis

Tower CO 2 flux measurements from 20 European grasslands in the EUROGRASSFLUX data set covering a wide range of environmental and management conditions were analyzed with respect to their ecophysiological characteristics and CO 2 exchange (gross primary production, P g, and ecosystem respiration, R e) using light-response function analysis. Photosynthetically active radiation ( Q) and top-soil temperature ( T s) were identified as key factors controlling CO 2 exchange between grasslands and the atmosphere at the 30-min scale. A nonrectangular hyperbolic light-response model P( Q) and modified nonrectangular hyperbolic light–temperature-response model P( Q, T s) proved to be flexible tools for modeling CO 2 exchange in the light. At night, it was not possible to establish robust instantaneous relationships between CO 2 evolution rate r n and environmental drivers, though under certain conditions, a significant relationship r n = r 0 e k T T s was found using observation windows 7–14 days wide. Principal light-response parameters—apparent quantum yield, saturated gross photosynthesis, daytime ecosystem respiration, and gross ecological light-use efficiency, ɛ = P g/ Q, display patterns of seasonal dynamics which can be formalized and used for modeling. Maximums of these parameters were found in intensively managed grasslands of Atlantic climate. Extensively used semi-natural grasslands of southern and central Europe have much lower production, respiration, and light-use efficiency, while temperate and mountain grasslands of central Europe ranged between these two extremes. Parameters from light–temperature-response analysis of tower data are in agreement with values obtained using closed chambers and free-air CO 2 enrichment. Correlations between light-response and productivity parameters provides the possibility to use the easier to measure parameters to estimate the parameters that are more difficult to measure. Gross primary production ( P g) of European grasslands ranges from 1700 g CO 2 m −2 year −1 in dry semi-natural pastures to 6900 g CO 2 m −2 year −1 in intensively managed Atlantic grasslands. Ecosystem respiration ( R e) is in the range 1800 < R e < 6000 g CO 2 m −2 year −1. Annual net ecosystem CO 2 exchange (NEE) varies from significant net uptake (>2400 g CO 2 m −2 year −1) to significant release (<−600 g CO 2 m −2 year −1), though in 15 out of 19 cases grasslands performed as net CO 2 sinks. The carbon source was associated with organic rich soils, grazing, and heat stress. Comparison of P g, R e, and NEE for tower sites with the same characteristics from previously published papers obtained with other methods did not reveal significant discrepancies. Preliminary results indicate relationships of grassland P g and R e to macroclimatic factors (precipitation and temperature), but these relationships cannot be reduced to simple monofactorial models.

Environmental controls on the CO2 exchange in north European mires

Net CO2 exchange measured under well-mixed atmospheric conditions in four different mires in Sweden and Finland were used to analyse which factors were controlling photosynthesis and respiration. The parameters of a light response function showed strong seasonal variations with similar behaviour for all mires. The half-monthly nighttime respiration rates in the central part of the growing season were about two times higher in the southernmost, warmest site, Fåje, as compared to the northernmost, coldest site, Kaamanen. However, Kaamanen had high photosynthesis rates, and this in combination with the long daylight periods in the middle of the summer caused Kaamanen to have the largest net ecosystem exchange (NEE) during the summer period. Fåje that showed the highest productivity had also the highest respiration and therefore, the lowest NEE during summer. Correlation between half-monthly components and different environmental variables showed the highest correlation between the components themselves. Thereafter came temperature except for Fåje where water table depth (WTD) explained most of the variance both for detrended and temperature-normalized components. All sites showed dependencies between WTD and the respective components during drying up periods. Temperature sensitivity was higher for productivity than for respiration indicating that CO2 uptake would increase during global warming.

Long-Term Dynamics of Production, Respiration, and Net CO2 Exchange in Two Sagebrush-Steppe Ecosystems

We present a synthesis of long-term measurements of CO2 exchange in 2 US Intermountain West sagebrush-steppe ecosystems. The locations near Burns, Oregon (1995–2001), and Dubois, Idaho (1996–2001), are part of the AgriFlux Network of the Agricultural Research Service, United States Department of Agriculture. Measurements of net ecosystem CO2 exchange (Fc) during the growing season were continuously recorded at flux towers using the Bowen ratio-energy balance technique. Data were partitioned into gross primary productivity (Pg) and ecosystem respiration (Re) using the light-response function method. Wintertime fluxes were measured during 1999/2000 and 2000/2001 and used to model fluxes in other winters. Comparison of daytime respiration derived from light-response analysis with nighttime tower measurements showed close correlation, with daytime respiration being on the average higher than nighttime respiration. Maxima of Pg and Re at Burns were both 20 g CO2 � m � 2 � d � 1 in 1998. Maxima of Pg and Re at Dubois were 37 and 35 g CO2 � m � 2 � d � 1 , respectively, in 1997. Mean annual gross primary production at Burns was 1 111 (range 475–1 715) g CO2 � � � � � � � � site could be a C sink or source, mostly depending on precipitation timing and amount. Total annual precipitation is not a good predictor of carbon sequestration across sites. Our results suggest that Fc should be partitioned into Pg and Re components to allow prediction of seasonal and yearly dynamics of CO2 fluxes. Resumen

Long-Term Dynamics of Production, Respiration, and Net CO2 Exchange in Two Sagebrush-Steppe Ecosystems

We present a synthesis of long-term measurements of CO2 exchange in 2 US Intermountain West sagebrush-steppe ecosystems. The locations near Burns, Oregon (1995-2001), and Dubois, Idaho (1996-2001), are part of the AgriFlux Network of the Agricultural Research Service, United States Department of Agriculture. Measurements of net ecosystem CO2 exchange (Fc) during the growing season were continuously recorded at flux towers using the Bowen ratio-energy balance technique. Data were partitioned into gross primary productivity (Pg) and ecosystem respiration (Re) using the light-response function method. Wintertime fluxes were measured during 1999/2000 and 2000/2001 and used to model fluxes in other winters. Comparison of daytime respiration derived from light-response analysis with nighttime tower measurements showed close correlation, with daytime respiration being on the average higher than nighttime respiration. Maxima of Pg and Re at Burns were both 20 g CO2 m2 d-1 in 1998. Maxima of Pg and Re at Dubois were 37 and 35 g CO2 m-2 d-1, respectively, in 1997. Mean annual gross primary production at Burns was 1 111 (range 475-1 715) g CO2 m-2 y-1 or about 30% lower than that at Dubois (1 602, range 963-2 162 g CO2 m-2 y-1). Across the years, both ecosystems were net sinks for atmospheric CO2 with a mean net ecosystem CO2 exchange of 82 g CO2 m-2 y-1 at Burns and 253 g CO-2 m-2 y-1 at Dubois, but on a yearly basis either site could be a C sink or source, mostly depending on precipitation timing and amount. Total annual precipitation is not a good predictor of carbon sequestration across sites. Our results suggest that Fc should be partitioned into Pg and Re components to allow prediction of seasonal and yearly dynamics of CO2 fluxes.

06/00176 Grassland bird response to harvesting switchgrass as a biomass energy crop: Roth, A. M. et al. Biomass and Bioenergy, 2005, 28, (5), 490–498

This study investigated the CO2 exchange over a 10-year period (2005–2014) inside and above a larch-dominant forest in the central Lena river basin, eastern Siberia. A wet-soil condition, such as that found in the active layer (seasonally thawed soil layer of upper permafrost), containing unusually high soil water close to saturation and partial surface waterlogging, was prolonged during the warm season of 2005–2009. In later years, the soil layer closer to the ground surface became dry (∼10% volumetric water content), although the deeper part remained relatively wet (∼30%). We quantitatively compared the whole forest and the understory CO2 exchanges to detect the separate effects of excessive soil waters on the overstory and understory vegetation. The conventional light and temperature response functions for half-hourly CO2 fluxes, that is, the net ecosystem exchange of daytime and night-time, respectively, were applicable to the understory observations. Comparison of the fitting parameters of the light response function at two levels revealed a smaller maximum net ecosystem exchange (NEE) under light saturation with a steep response under weak light conditions for the understory. The CO2 exchanges at the understory increased from the wet-soil period to the drying soil period by 46% (1.3 g C m−2 d−1) of gross primary production (GPP) and 29% (1.2 g C m−2 d−1) of ecosystem respiration (ER), while no trend was found in the ecosystem scale fluxes. These increases were due to an increasing understory biomass, changes in plentiful light and soil water in the inside-canopy environments, and enhanced turbulent mixing. The decline in the larch contribution could be compensated for by the understory growth and the remaining wetness of the active layer, which indicated that the interactions between the larch and the understory supported the stability of carbon cycles in this forest ecosystem.

Experimental analysis of flux footprint for varying stability conditions in an alpine meadow

Eddy-covariance measurements, which were performed in the summer 2003 in an alpine grassland site, were used to determine the footprint of CO 2 turbulent fluxes and to investigate its dependence on stability. The analysis is based on the spatial variability of carbon sinks generated by the difference in the time of the cutting of two concentric portions of the footprint, located outside and inside the fence surrounding the eddy tower, at approximately 30 m distance. Due to this time difference, turbulent flux measurements were performed both in homogeneous (BEFORE and AFTER cutting) and heterogeneous (BETWEEN cuts) condition of spatial sink distribution. The analysis is based exclusively on daytime measurements. Half-hourly records ranked in two stability classes were used to calculate light response functions separately for three 10-day periods (BEFORE, BETWEEN and AFTER cutting). The contribution of the area inside the fence to the total flux was determined for different photosynthetic photon flux density (PPFD) values considering the BETWEEN cuts light response curve as a weighted average of the BEFORE and AFTER ones. The weight of the BEFORE cutting light response curve has been analytically determined and corresponds to the flux fraction which originates from inside the fence. The experimental estimates of the relative importance of the area inside the fence produced values ranging from 30%, during stable conditions, and up to 80% during unstable conditions. These values derived from observations were later compared with the predictions of three analytical footprint models. One of the models systematically underestimated experimental observations, while observations concurred with the other two, particularly in moderately unstable conditions.

Estimating photosynthetic light-use efficiency using the photochemical reflectance index: variations among species.

Recent studies have shown that the photochemical reflectance index (PRI), derived from narrow waveband reflectance at 531 and 570 nm, can be used as a remote measure of photosynthetic light-use efficiency (LUE). However, uncertainty remains as to the consistency of the relationship between PRI and LUE across species. In this study we examined the relationship between the PRI and various photosynthetic parameters for a group of species with varying photosynthetic capacity. At constant irradiance, for the species group as a whole, the PRI was well correlated with LUE (r2=0.58) and with several other photosynthetic parameters, but best correlated with the ratio of carotenoids to chlorophylls contents (Caro / Chl). Despite the interspecific trends observed, determination of light response functions for the PRI in relation to photosynthetic parameters revealed that species-specific relationships were clearly stronger. For example, r2>0.90 for species-level PRI / LUE relationships. Also, the species-specific light-response data show that the magnitude of the PRI can be related to the magnitude of the saturated irradiance and the rate of CO2 uptake. As demonstrated here, a light response function provides a simple yet precise approach for characterising the relationship between the PRI and photosynthetic parameters, which should assist with improved evaluation of the usefulness of the PRI as a generalised measure of LUE.

Evaluation of pixelated NaI(Tl) detectors for PET

A new detector design using pixelated NaI(Tl) crystals has been evaluated for use in positron emission tomography (PET). This detector uses 4/spl times/4/spl times/30 mm/sup 3/ NaI(Tl) pixels coupled via a lightguide (1.4 cm thick) to an array of 39 mm diameter photomultiplier tubes (PMTs) in an Anger detector design. Our measurements show that the high light output of NaI(Tl) leads to a good discrimination of the 4 mm NaI(Tl) pixels for varying integration times of 220, 120, and 60 ns. The crystal-to-crystal variation in the measured energy for a central group of nine crystals is 1.1%, while the average energy resolution at 511 keV varies from 8.6% to 10.7% as the integration time decreases from 220 to 60 ns. Additionally, the measured light response function (LRF) of the pixelated detector is much narrower than that of the continuous, curve-plate plate C-PET detectors, even though the pixelated crystal is thicker. High count-rate simulations for a whole-body scanner with pixelated NaI(Tl) detectors but based on the C-PET geometry (diameter of 90 cm and an axial field-of-view of 25 cm) predict more than a doubling of the peak NEC rate over the C-PET scanner with curved, continuous detectors. This is due to the increased sensitivity of thicker crystals and reduced deadtime achieved by the pixelated detector design. Thus, a pixelated NaI(Tl) detector based scanner offers significant improvements in both the spatial resolution and count-rate performance over the current whole-body C-PET scanner.

CMiCE: a high resolution animal PET using continuous LSO with a statistics based positioning scheme

Objective: Detector designs for small animal scanners are currently dominated by discrete crystal implementations. However, given the small crystal cross-sections required to obtain very high resolution, discrete designs are typically expensive, have low packing fraction, reduced light collection, and are labor intensive to build. To overcome these limitations we have investigated the feasibility of using a continuous miniature crystal element (cMiCE) detector module for high resolution small animal PET applications. Methods: The detector module consists of a single continuous slab of LSO, 25×25 mm 2 in exposed cross-section and 4 mm thick, coupled directly to a PS-PMT (Hamamatsu R5900-00-C12). The large area surfaces of the crystal were polished and painted with TiO 2 and the short surfaces were left unpolished and painted black. Further, a new statistics based positioning (SBP) algorithm has been implemented to address linearity and edge effect artifacts that are inherent with conventional Anger style positioning schemes. To characterize the light response function (LRF) of the detector, data were collected on a coarse grid using a highly collimated coincidence setup. The LRF was then estimated using cubic spline interpolation. Detector performance has been evaluated for both SBP and Anger based decoding using measured data and Monte Carlo simulations. Results: Using the SBP scheme, edge artifacts were successfully handled. Simulation results show that the useful field of view (UFOV) was extended to ∼22×22 mm 2 with an average point spread function of ∼0.5 mm full width of half maximum (FWHM PSF). For the same detector with Anger decoding the UFOV of the detector was ∼16×16 mm 2 with an average FWHM PSP of ∼0.9 mm. Experimental results yielded similar differences between FOV and resolution performance. FWHM PSF for the SBP and Anger based method was 1.4 and 2.0 mm, uncorrected for source size, with a 1 mm diameter point source, respectively. Conclusion: A continuous detector module with an average FWHM PSF approaching one millimeter has been built and tested. Furthermore, the results demonstrate that our SBP scheme yields improved performance over traditional Anger techniques for our cMiCE detector.

Slotted surface treatment of position-sensitive NaI(Tl) detectors to improve detector performance

We have investigated cutting slots in the entrance surface of 1-in-thick NaI(Tl) crystals, which are used in large position-sensitive detectors, to narrow the detector light response function (LRF). The LRF describes the spread of light from the crystal to the photomultiplier tube (PMT) array. A narrow LRF will, therefore, reduce pileup effects in these detectors at high count rates, and so improve the count-rate performance of NaI(Tl)-based positron emission tomography (PET) scanners. The slot pitch was kept fixed at 5 mm to take into consideration the time required to cut the slots and the slight reduction in sensitivity for the slotted detector. Slots will narrow the LRF and simultaneously make the detector response to position changes discrete in nature. Our investigation focuses on optimizing the slot depth and PMT readout array to achieve a narrow LRF with a detector spatial resolution close to that of the 5-mm slot pitch. Our measurements and simulations show the narrowing of the LRF for large two-dimensional (2-D) detectors with increasing slot depth. However, using 50-mm PMTs the variation of the point spread function between adjacent slots, together with the discrete 5-mm sampling, results in a worsening of the detector spatial resolution for a slotted detector compared to a similar unslotted detector. Simulations show that using smaller 39-mm PMTs will result in better position sampling and a reduction in the variation of the point spread function (PSF) in between adjacent slots. The 2-D detector with 15-mm-deep slots and 39-mm. PMTs results in a detector spatial resolution which is similar to that of an unslotted detector. However, the LRF of this slotted detector is significantly narrower (about 35%), thus reduced event pileup would be expected at high count rates.

Remote sensing of canopy light use efficiency using the photochemical reflectance index: Model and sensitivity analysis

A growing number of studies have shown that reflectance changes at 531 nm, associated with the xanthophyll cycle and the related thylakoid energisation are widespread among plant species. The photochemical reflectance index (PRI), derived from narrow band reflectance at 531 and 570 nm has been related with some success to photosynthetic light use efficiency (LUE). Such a relationship would enable the estimation of stand photosynthesis from remotely sensed data. However, canopy PRI is an integral of the component leaf response weighted by the strength of the signal from each leaf to the sensor. This analysis investigates the extent to which canopy structure, view, and illumination angles are likely to influence the measured canopy PRI. A one-dimensional ray tracing radiative transfer model was used to estimate light distribution within a canopy and the dynamic response of individual foliar elements, based on a published relationship between PRI and LUE and a simple photosynthetic light response function. The model estimated the LUE of the canopy, based on both incident and absorbed light, and reflectance of the canopy at the desired wavelengths and hence the canopy PRI. A range of solar zenith, leaf area index (LAI), leaf angle distributions (LAD), and soil types were used to determine the likely influence on measured canopy PRI. The results show a positive correlation between PRI and LUE variation at canopy scale. However, the index shows a greater variation of view angle than most vegetation indices. The index is strongly influenced by varying soil background for LAI<3. At large viewing angles (>30°) the index is also sensitive to LAD. Correction for Rayleigh scattering is necessary to relate the index to ground measured PRI. Results show that the PRI value is most sensitive to changes in LAI. Utilisation of the relationship to predict or improve estimates of canopy LUE based on either absorbed or incident light will require an independent estimate of LAI change between dates/locations of in situ measurements and of remote sensing observations.

On-beam calibration of the Δ E(Si)–Sci/PD charged particle telescope

The reaction products emitted in the 14 N(45 A MeV )+( CH 2/ CD 2) interactions are identified by a Δ E(Si)– E(Scintillator/Photodiode) telescope by the conventional Δ E– E method. The position of “jumps” in the amplitude of the photodiode signal for ions passing through the scintillator (Sci) is used to calibrate on-beam both the Δ E and the Sci/PD scales in MeV. The accuracy of an absolute energy calibration is better than 2.3% and 1.8% for CsI(Tl) and GSO(Ce) detectors, respectively. It is defined mostly by the correctness of the range-energy relations of ions in the Si and Sci crystals. The light response function, L( E, Z, A), of isotopes up to Z( A)=8(16) in the range of energy ∼(2.5–60) A MeV is extracted. The effects of doping concentration and pulse shaping on the light response are analyzed. The validity of the existing empirical light-energy relations is checked in a wide interval of ion energies and a new power law relation is proposed. Calculations of the response function based on the Murray–Mayer model are found to be in excellent agreement with experimental data for the CsI(Tl) crystal.

Investigation of bias-free positioning estimators for the scintillation cameras

Bias-free positioning estimators for scintillation cameras are investigated. A linear correlation coefficient (LCC) and Chi square error (CSE) method are evaluated with respect to linearity and spatial resolution performance. The LCC method uses correlation information between the true function (i.e., light response function) and measured data in mapping an event characterization vector to the associated position. The CSE method estimates the position where the Chi square error between two functions becomes minimized. In order to determine true statistics as a function of position, the light response function (LRF) was estimated based on sample measurements by using a cubic spline interpolation technique which provides smooth first-order and continuous second-order derivative of the LRF. Both methods have superior linearity properties compared to the weighted centroid. Each method call be considered as a bias-free positioning estimator within the effective field of view of the detector. The spatial resolution performance of the CSE method is /spl sim/7% and /spl sim/16% better than the weighted centroid method for a 16- and 25-mm-thick crystal, respectively. The spatial resolution performance of the LCC method is comparable to that of the weighted centroid method.